Method for binding particles to fibers using reactivatable binders

ABSTRACT

Particles, such as superabsorbent particles, are bound to fibers, such as cellulosic fibers, by a binder that has a volatility less than water. The binder has a functional group capable of forming a hydrogen bond with the fibers, and a functional group that is capable of forming a hydrogen bond or a coordinate covalent bond with the particles. The binder is activated or reactivated by addition of heat, liquid, or mechanical energy. Therefore, fibers treated with binder may be shipped to a distribution point before particles are bound to the fibers. The binder may be a polymeric binder selected from the group consisting of polyethylene glycol, polypropylene glycol, poly(caprolactone) diol, polyacrylic acid, polyamides and polyamines. The polymeric binder has a hydrogen bonding functionality or coordinate covalent bond forming functionality on each repeating unit of the polymeric binder. Alternatively, the binder may be a non-polymeric organic binder that includes a functionality such as a carboxylic acid, an aldehyde, an alcohol, an amino acid, an amide, and an amine, wherein there are at least two such functionalities on the molecule. Particles attached to the fibers in this manner are firmly adhered and are not easily dislodged. Fibrous products produced by this method include fibers to which particles are bound, and fibers which have been treated with the binder but to which particles are not bound.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns reactivatable polymeric and non-polymericbinders for fibers and the use of such binders in binding particles tofibers. In particular embodiments, it concerns binding superabsorbentparticles to cellulosic fibers which may then be used, for example, tomake absorbent fibers that are incorporated into cellulosic products.

2. General Discussion of the Background

Superabsorbent polymers have been developed in recent years that arecapable of absorbing many times their own weight of liquid. Thesepolymers, which are also known as water insoluble hydrogels, have beenused to increase the absorbency of sanitary products such as diapers andsanitary napkins. Superabsorbent polymers are often provided in the formof particulate powders, granules, or fibers that are distributedthroughout absorbent cellulosic products to increase the absorbency ofthe product. Superabsorbent particles are described, for example, inU.S. Pat. No. 4,160,059; U.S. Pat. No. 4,676,784; U.S. Pat. No.4,673,402; U.S. Pat. No. 5,002,814; and U.S. Pat. No. 5,057,166.Products such as diapers that incorporate absorbent hydrogels are shownin U.S. Pat. No. 3,669,103 and U.S. Pat. No. 3,670,731.

One problem with the use of superabsorbents is that the superabsorbentmaterial can be physically dislodged from the cellulosic fibers of anabsorbent product. Separation of the superabsorbent from its substratereduces the absorbency of the product and diminishes the effectivenessof the superabsorbent material. This problem was addressed in EuropeanPatent Application 442 185 A1, which discloses use of a polyaluminumchloride binder to bind an absorbent polymer to a fibrous substrate. Thepolyaluminum binder, however, suffers from the drawback of being aninorganic product that is not readily biodegradable. Moreover, thatEuropean patent does not offer any guidance for selecting binders otherthan polyaluminum chloride that would be useful in binding absorbentparticles.

A method of immobilizing superabsorbents is disclosed in U.S. Pat. No.4,410,571 in which a water swellable absorbent polymer is converted to anon-particulate immobilized confluent layer. Polymer particles areconverted to a coated film by plasticizing them in a polyhydroxy organiccompound such as glycerol, ethylene glycol, or propylene glycol. Thesuperabsorbent assumes a non-particulate immobilized form that can befoamed onto a substrate. The individual particulate identity of thesuperabsorbent polymer is lost in this process. The confluent nature ofthe superabsorbent material can also result in gel blocking, in whichabsorption is diminished as the water swollen polymers block liquidpassage through the film layer.

U.S. Pat. No. 4,412,036 and U.S. Pat. No. 4,467,012 disclose absorbentlaminates in which a hydrolyzed starch polyacrylonitrile graft copolymerand glycerol mixture is laminated between two tissue layers. The tissuelayers are laminated to each other by applying external heat andpressure. The reaction conditions form covalent bonds between the tissuelayers that firmly adhere the tissue layers to one another.

Numerous other patents have described methods of applying binders tofibrous webs. Examples include U.S. Pat. No. 2,757,150; U.S. Pat. No.4,584,357; and U.S. Pat. No. 4,600,462. Such binders are not describedas being useful in binding particulates, such as superabsorbentparticles, to fibers. Yet other patents disclose crosslinking agentssuch as polycarboxylic acids that form covalent intrafiber bonds withindividualized cellulose fibers, as in European Patent Application440472 A1; European Patent Application 427 317 A2; European PatentApplication 427 316 A2; and European Patent Application 429 112 A2. Thecovalent intrafiber bonds are formed at elevated temperatures andincrease the bulk of cellulose fibers treated with the crosslinker. Thecovalent bonds between the fibers produce a pulp sheet that is moredifficult to compress to conventional pulp sheet densities than in anuntreated sheet. Any covalent crosslink bonds that form between thefibers and particles occupy functional groups that would otherwise beavailable for absorption, hence absorption efficiency is decreased.

Many different types of particles other than superabsorbents may beadded to fibers for different end uses. Antimicrobials, zeolites andfire retardants are but a few examples of particles that are added tofibers. It would be advantageous to provide a method of attachingparticles that could be accommodated to the many different particleneeds of end users. Moreover, it would be advantageous to reduceparticulate waste in the attachment process, and simplify shipment offiber products that require particulate addition.

Accordingly, it is an object of this invention to provide an improvedmethod of binding particulates to fibers which can be customized toeasily allow different end users of the products to bind different kindsof particles to the fibers.

It is also an object of this invention to provide an improved method ofbinding particulates, such as superabsorbent particles, to fibers.

It is another object to provide an improved method of bindingparticulates such that they can be distributed throughout a fibrousproduct in a desired distribution and without necessarily being confinedto the surface of a product.

Another object of the invention is to provide improved fiber andabsorbent products in which particulates are firmly bound to cellulosefibers such that the particles are less likely dislodged by mechanicalforces.

Yet another object of the invention is to provide an improved particlebinder that is environmentally compatible and more easily biodegradable.

Even yet another object is to provide such a product that has improvedprocessing characteristics, such as ease of densification.

Finally, it is an object of the invention to bind a broad variety ofparticles to many different kinds of fibers using an improved, simpleand versatile binding process that limits particle waste.

SUMMARY OF THE INVENTION

The foregoing and other objects are achieved by providing fibers withhydrogen bonding functional sites, and applying to the fibers a binderthat has a volatility less than water. The binder has a functional groupthat forms a hydrogen bond with the fibers, and a functional group thatis also capable of forming a hydrogen bond or a coordinate covalent bondwith particles that have a hydrogen bonding or coordinate covalentbonding functionality. The binder attaches the particles to the fibers,and forms a bond that has been found to be resistant to mechanicaldisruption. A significant advantage of these binders is that the binderscan be present on fibers in an inactive state, then later activated orreactivated to bind particles to the fibers.

Liquid binders (which includes aqueous solutions of solid binders orneat liquids) can be placed on the fibers, air dried, and laterreactivated by moistening the fibers. Alternatively, a dry solid bindermay be added to the fibers and later activated by addition of a liquid.An inactive binder can also be activated by applying kinetic energy tothe fibers after the binder and fibers reach an equilibrium moisturecontent with the atmosphere (hereinafter referred to as "air drying").Kinetic energy can be applied to the fibers, for example, bymechanically agitating the binder and fibers. In yet other embodiments,the binder may be activated or reactivated by heating the fibers afterapplying the binder to the fibers.

The capacity for activation or reactivation allows the binder to beapplied to the fibers, which are then shipped to distribution pointswith the binder in an inactive form. The binder is then activated at thedistribution point where particles are added to the fibers and boundthereto. As used herein, binder "activation" includes both activation ofpreviously inactive binders (such as solid binders in the absence ofliquid) or reactivation of previously active binders (such as a liquidbinder that has been air dried).

Another advantage of the present invention is that the binder can beactivated or reactivated in a pattern that corresponds to a desireddistribution of particles in fibrous material. A reactivation liquid,for example, can be applied to the areas of a diaper that will beinitially moistened by urine during use. Superabsorbent particles can beadded to activated area of the diaper and adhered almost exclusively inthose areas where initial urine absorption is required. Targetedactivation of binder allows particles to be efficiently and economicallyattached to the fibers, with reduced particle wastage. Moreover,targeted binder activation and particle adherence increases theabsorptive efficiency of the product by diminishing excessive wicking ofliquid within the plane of an absorptive product.

The fibers of the present invention may have particles bound to thefibers with a polymeric or non-polymeric binder. The polymeric bindermay be polypropylene glycol (PPG), polyethylene glycol (PEG),polyacrylic acid (PAA), a poly(caprolactone) diol, polyamide, apolyamine, and copolymers thereof (for example a polypropyleneglycol/polyethylene glycol copolymer), wherein the polymeric binder hasa hydrogen bonding functionality or coordinate covalent bond formingfunctionality on each repeating unit of the polymeric binder. In thepresent invention, the non-polymeric binder has a volatility less thanwater, a functional group that forms hydrogen bonds or coordinatecovalent bonds with the particles, and a functional group that formshydrogen bonds with the cellulose fibers. The non-polymeric binder is anorganic binder, and preferably includes a functionality selected fromthe group consisting of a carboxylic acid, an aldehyde, an alcohol, anamino acid, an amide, and an amine, wherein there are at least twofunctionalities on the molecule selected from this group, and the twofunctionalities are the same or different. Examples of such bindersinclude polyols, polyamines (a non-polymeric organic binder with morethan one amine group), polyamides (a non-polymeric organic binder withmore than one amide group), polycarboxylic acids (a non-polymericorganic binder with more than one carboxylic acid functionality), apolyaldehyde (a non-polymeric organic binder with more than onealdehyde), amino alcohols, hydroxy acids and other binders. Thesebinders have functional groups that are capable of forming the specifiedbonds with the particles and fibers.

More preferably, the organic non-polymeric binder is selected from thegroup consisting of glycerin, glyoxal, ascorbic acid, urea, glycine,pentaerythritol, a monosaccharide or a disaccharide, citric acid,tartaric acid, dipropylene glycol, and urea derivatives such as DMDHEU.Suitable saccharides include glucose, sucrose, lactose, ribose,fructose, mannose, arabinose, and erythrose. Each of these preferredbinders is a non-polymeric molecule that has a plurality of hydrogenbonding functionalities that permit the binder to form hydrogen bonds toboth the fibers and particles. Particularly preferred binders includethose that can form five or six membered rings, most preferably sixmembered rings, with a functional group on the surface of the particle.

The fibrous material may be cellulosic or synthetic fibers that arecapable of forming hydrogen bonds with the binder, while the particlesform hydrogen bonds or coordinate covalent bonds with the binder. Thisbinder system secures particles to fibers unexpectedly well. A superiorfibrous product is therefore produced that has improved absorbentproperties as compared to unbound or covalently bound particles.Formation of the noncovalent bond allows production of a fiber productthat is easily manufactured and a web that is easily densified, and thatis readily biodegradable and disposable.

In one preferred embodiment, an absorbent product comprises a fibrouscellulosic mat that contains superabsorbent hydrogel particles inparticulate form. The superabsorbent particles form hydrogen bonds orcoordinate covalent bonds with the binder, depending upon the binder,while the binder in turn forms hydrogen bonds with the hydroxyl groupsof the cellulose fibers. These noncovalent, relatively flexible bondsbetween the binder and particles maintain the particles in contact withthe fibers, and resist dislodgement of the particles by mechanicalforces applied to the mat during manufacture, storage or use. The bindermay suitably be present in an amount of from about 3 to 80 percent ofthe total weight of the product, while the particles bound to the binderof the present invention (via hydrogen/coordinate covalent bonds) maysuitably be present in an amount of 0.05 to 80 percent, preferably 1 to80 percent or 5 to 80 percent by weight. An especially suitable range ofbinder is 3 to 40 percent by weight, or 3 to 25 percent by weight, whilea particularly suitable range of such particles is 5 to 40 percent byweight. A preferred weight ratio of particle to binder is 2:1 to 4:1. Anexample of a suitable particle is a superabsorbent polymer such as astarch graft polyacrylate hydrogel fine or larger size particle such asa granule, which forms hydrogen bonds with the binder. The binder alsoforms hydrogen bonds with the hydroxyl groups of the cellulose, therebysecurely attaching the superabsorbent particles to the fibers.

The present invention also includes a method of binding particles tofibers wherein the particles are insoluble in the binder and thereforeretain their solid particulate form following binding. The particleshave functional groups that can form hydrogen bonds or coordinatecovalent bonds with the binder, and the binder in turn is capable offorming hydrogen bonds to the fibers.

In especially preferred embodiments, the fibers are cellulosic and theparticles are superabsorbent particles that are bound to the binder byhydrogen bonds. The fibers may also be continuous or discontinuoussynthetic or natural fibers having a hydrogen bonding functional groupthat hydrogen bonds with the binder. The binder is suitably applied tothe fibers in an amount of at least 3 percent, and preferably no morethan 80 percent, by total weight of the particle, fiber and binder. Theparticles may be bound to the fibers at less than 150° C. or without anyexternal application of heat at ambient temperature (e.g., about 25°C.). Particles may also be bound in the absence of any externalapplication of pressure, or in the absence of external heat andpressure.

In some embodiments the binder is associated with the fibers as a solid(for example, a dry powder or a dried liquid), and the fibers contain atleast 7 percent water by weight when the binding step is performed. Thislevel of moisture in the fibers provides sufficient mobility ofreactants to allow the particles and fibers to bind well to each other.When a liquid binder is used (for example, glycerin or a solution ofglycine powder), the fibers suitably contain at least about 0.5 percentwater by weight. A solid binder is suitably used with fibers having 0.5percent water by weight if the binder is heated above its melting pointto liquefy it. A solid binder may be thermoplastic or meltable, suchthat it can be heated above its melting point and then cooled to fusefibers to each other. The thermoplastic properties of the binder canalso provide additional mechanical adherence between the particles andfibers. In some embodiments, a thermoplastic binder such as urea may beemployed which can adhere particles both thermoplastically and withhydrogen bonding.

The invention also includes the products produced by any of the methodsdescribed herein.

The foregoing and other features and advantages of the invention willbecome more apparent from the following detailed description andaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a structure into which fibers of thepresent invention are incorporated with attached particles, the fibersbeing in the form of an illustrated absorbent pad.

FIG. 2 represents a partial sectional view of the pad of FIG. 1.

FIG. 3 illustrates a plan view of a bandage incorporating fibers of thepresent invention.

FIG. 4 is a sectional view of the bandage of FIG. 5, taken along line4--4 of FIG. 3.

FIG. 5 is a perspective view of an absorbent structure of fibers of thepresent invention.

FIG. 6 is a cross-sectional view of the structure of FIG. 5, taken alongline 6--6 of FIG. 5.

FIG. 7 is a plan view of a feminine hygiene appliance incorporatingfibers of the present invention.

FIG. 8 is a sectional view of the appliance of FIG. 9 taken along line8--8 of FIG. 7.

FIG. 9 is a plan view of a disposable diaper including a core of fibersof the present invention.

FIG. 10 is a vertical sectional view of the diaper of FIG. 9.

FIG. 11 is a view of an enlarged fiber with particles bonded to thefiber with the binders of the present invention.

FIG. 12 is a schematic view of a cellulose mat with particles bound toall its surfaces and throughout its depth.

FIG. 13 is a photomicrograph of particles adhered to fibers with anascorbic acid binder.

FIGS. 14A, 14B, 14C and 14D are photomicrographs of particles bound tofibers with lactose.

DETAILED DESCRIPTION OF SEVERAL PREFERRED EMBODIMENTS OF THE INVENTIONFiber Characteristics

The present invention includes a method of binding particles to fibers,and the product produced by that method. In particularly preferredembodiments, the product is a cellulosic or synthetic fiber to whichsuperabsorbent hydrogel polymer particles are adhered by a binder, andabsorbent products made therefrom. Suitable fibers include wood pulpfibers, which can be obtained from well known chemical processes such asthe kraft and sulfite processes. In these processes, the best startingmaterial is prepared from long fiber coniferous wood species, such aspine, douglas fir, spruce and hemlock. Wood pulp fibers can also beobtained from mechanical processes, such as ground wood, mechanical,thermomechanical, chemimechanical, and chemithermomechanical pulpprocesses. The fibers are preferably elongated, for example having alength to width ratio of about 10:1 to 5:1.

The fibers of the present invention also include fibers that arepretreated prior to the application of a binder to the fibers asexplained below. This pretreatment may include physical treatment, suchas subjecting the fibers to steam or chemical treatment, such ascross-linking the fibers. Although not to be construed as a limitation,examples of pretreating fibers include the application of fireretardants to the fibers, such as by spraying the fibers with fireretardant chemicals. Specific fire retardant chemicals include, by wayof example, sodium borate/boric acid, urea, urea/phosphates, etc. Inaddition, the fibers may be pretreated with surfactants or otherliquids, such as water or solvents, which modify the surface of thefibers. Other pretreatments include exposure to antimicrobials orpigments.

The fibers may also be pretreated in a way which increases theirwettability. For example, natural fibers may be pretreated with a liquidsodium silicate, as by spraying the fibers with this material, forpretreatment purposes. Wettability of the surface of fibers is alsoimproved by subjecting the fibers to a corona discharge pretreatment inwhich electrical current is discharged through the fibers in aconventional manner. In the case of both synthetic fibers and wood pulpfibers, corona discharge pretreatment results in an oxygen functionalityon the surface of the fibers, making them more wettable and morebondable. The fibers may also be pretreated with conventionalcross-linking materials and may be twisted or crimped, as desired.Pretreating cellulose fibers with chemicals which result in lignin orcellulose rich fiber surfaces may also be performed in a conventionalmanner.

Bleaching processes, such as chlorine or ozone/oxygen bleaching may alsobe used in pretreating the fibers. In addition, the fibers may bepretreated, as by slurrying the fibers in baths containing antimicrobialsolutions (such as solutions of antimicrobial particles as set forthbelow), fertilizers and pesticides, and/or fragrances and flavors, forrelease over time during the life of the fibers. Fibers pretreated withother chemicals, such as thermoplastic and thermoset resins may also beused. Combinations of pretreatments may also be employed with theresulting pretreated fibers then being subjected to the application ofthe binder coating as explained below.

Ground wood fibers, recycled or secondary wood pulp fibers, and bleachedand unbleached wood pulp fibers can be used. Details of the productionof wood pulp fibers are well known to those skilled in the art. Thesefibers are commercially available from a number of companies, includingWeyerhaeuser Company, the assignee of the present invention.

The fibers can also be any of a variety of other natural or syntheticfibers, however, all of the fibers to which particles are attached inaccordance with the present invention include a hydrogen bondingfunctionality. This does not preclude the blending of such fibers withfibers lacking this characteristic. However, the fibers lacking ahydrogen bonding functionality will not have particles bonded theretowith the strength of the bonds that would be present if the fibers had ahydrogen bonding functionality.

A hydrogen bond is an intermolecular force that occurs between hydrogenatoms that are covalently bonded to small, strongly electronegativeelements (such as nitrogen and oxygen) and nonbonding electron pairs onother such electronegative elements. A hydrogen bonding functionality isa functional group that contains an oxygen or nitrogen atom, for examplehydroxyls, carboxyls, ethers, esters, epoxides, carbonyls, amines,urethanes and others, that is capable of forming a hydrogen bond. Theorbitals of the nonbonding electron pairs on the oxygen or nitrogenoverlap with the relatively empty 1s orbital of the hydrogen covalentlybonded to another nitrogen or oxygen atom. The 1s orbital of thehydrogen is relatively empty due to the unequal sharing of the electronsin the covalent bond between it and the small electronegative atom(oxygen or nitrogen) to which it is bound.

Specific examples of natural fibers that contain a hydrogen bondingfunctionality include chopped silk fibers, wood pulp fibers, bagasse,hemp, jute, rice, wheat, bamboo, corn, sisal, cotton, flax, kenaf, peatmoss, and mixtures thereof. Suitable synthetic fibers with hydrogenbonding functionalities include acrylic, polyester, carboxylatedpolyolefins, rayon and nylon. The hydrogen bonding functionality is anester in acrylic fibers and a carboxylic acid in carboxylated polyolefinfibers, an ester in polyester, an amide in nylon, and a hydroxyl inrayon. Polyethylene and polypropylene would be unsuitable fibers for usein particle to fiber bonding in accordance with the present inventionbecause they include only carbons and hydrogens without any oxygens ornitrogens that can participate in hydrogen bonds.

For purposes of convenience, and not to be construed as a limitation,the following description proceeds with reference to the treatment ofindividual chemical wood pulp fibers. The fibers are individualized, forexample by defiberization in a hammermill. Such individualized fibersare conventionally formed into a mat, and are commercially available,for example as NB 416 from the Weyerhaeuser Company. Another suitablecellulosic mat would include Rayfloc JLD from ITT Rayonier. Thecellulose fibers may be in the form of a cellulosic web or loosecellulose fibers.

Particle Characteristics

In accordance with the present invention, particles are added to the matto give it desired properties, such as increased absorbency,abrasiveness, or antimicrobial activity. The particle can be anyparticulate material that has the desired property and which is capableof forming hydrogen bonds or coordinate covalent bonds with the binder.Hydrogen bonds can be formed, as discussed above, by particles thatcontain functional groups having an oxygen or nitrogen. Coordinatecovalent bonds, in contrast, are formed by donation of a lone pair ofelectrons on one atom to an empty orbital of another atom. Coordinatecovalent bonds differ from covalent bonds in that covalent bonds areformed by a pair of electrons wherein one of the electrons is donatedfrom each of the atoms that participate in the bond. Particles can formcoordinate covalent bonds if they have an empty p or d or f orbital thatis capable of accepting a pair of electrons from the binder.

A coordinate covalent bond occurs between a donor compound that has alone pair of electrons to donate to the bond, and an acceptor that hasan empty orbital to accept the lone pair of electrons from the donor.According to the Aufbau and Pauli principles, electrons occupy the lobesof atomic orbitals one at a time with a maximum of two electrons (withopposite spins) per lobe. The most basic orbital is the s orbital, whichis available for bonding the elements in the first row of the periodictable. In the second row of the periodic table, electrons fill first the2s orbital of Li and Be, but metals in Groups IA and IIA do not havesufficient affinity for electrons to participate in coordinate covalentbonding. Beginning with column IIIA (boron), the three p orbitalsparticipate in coordinate covalent bonding and the lobes of the porbitals begin to fill. Boron has one electron in one of the 2p orbitalsthus leaving the other two p orbitals empty and available for coordinatecovalent bonding. An example of a coordinate covalently bonded boroncontaining particle is boric acid, which is used as an astringent,antiseptic and fire retardant. Boric acid is shown below wherein theboron is coordinate covalently bonded to a polypropylene glycol (PPG)binder. ##STR1## The next element, carbon, usually hybridizes to haveone electron in the 2s orbital and the three remaining electrons aresingly placed in the three p orbitals. This leaves no lobes empty forcoordinate covalent bonding and electron additions proceeding furtheracross that row of the periodic table also leave no lobes empty. Hence,boron is the only element in the second row of the periodic table thatis capable of forming coordinate covalent bonds.

Next the third row begins to fill, and the two 3s electrons fill firstin sodium and magnesium, but these metals in groups IA and IIA do notform coordinate covalent bonds as discussed above. Then aluminum, likeboron, places one electron in one of the 3p lobes, and the two other 3plobes are empty and available for coordinate covalent bonding. The sametrends continue across the third row, but the third row elements alsohave available five 3d lobes so the potential for coordination bondingexists even though 3p orbitals are occupied in the third row. Hence, Al,P, S, and Cl are capable of accepting a pair of electrons from anelectron pair donor to form a coordinate covalent bond. An example ofthis is found in the bonding in PCl₅, aluminum trihydrate, orphosphorous pentasulfide. A phosphorous pentasulfide particle can beused to increase flammability of a product, while aluminum trihydrate isa fire retardant. An example of a coordinate covalently bonded aluminumcompound is ##STR2## wherein aluminum trihydrate is coordinatecovalently bonded to a polypropylene glycol (PPG) polymer.

In the next row, the 4s orbital is filled first, then the 3d lobes beginto fill--one electron per lobe until all have a single then a secondelectron to each lobe until all lobes are filled. However, 4p and 4forbitals are also available, hence many of the transition elements arecapable of forming coordinate covalent bonds.

The elements that have empty orbitals that participate in coordinatecovalent bonding include all those except the metals (which excludeshydrogen) in groups IA and IIA, and C, N, O, F, Ne and He. Especiallypreferred particles contain boron, aluminum, iron, rhodium, osmium,platinum, and palladium, particularly boron. Examples of particles thatare capable of coordinate covalent bonding are aluminum trihydrate,antimony oxide, arsenic disulfide, bismuth aluminate, bismuth iodideoxide, bismuth phosphate, bismuth subcarbonate, bismuth subgallate,cadmium salycilate, chromic carbonate, chromic hydroxide, chromic oxide,and chromic phosphate. All of the polymeric binders of the presentinvention (PPG, PAA, poly(caprolactone) diol, polyamide and polyamine)are capable of donating a lone pair of electrons from an oxygen ornitrogen to form a coordinate covalent bond with a suitable particlethat has an empty orbital for coordinate covalent bonding.

Superabsorbent Particles

In one disclosed embodiment the added particles are superabsorbentparticles, which comprise polymers that swell on exposure to water andform a hydrated gel (hydrogel) by absorbing large amounts of water.Superabsorbents are defined herein as materials that exhibit the abilityto absorb large quantities of liquid, i.e. in excess of 10 to 15 partsof liquid per part thereof. These superabsorbent materials generallyfall into three classes, namely starch graft copolymers, crosslinkedcarboxymethylcellulose derivatives and modified hydrophilicpolyacrylates. Examples of such absorbent polymers are hydrolyzedstarch-acrylonitrile graft copolymer, a neutralized starch-acrylic acidgraft copolymer, a saponified acrylic acid ester-vinyl acetatecopolymer, a hydrolyzed acrylonitrile copolymer or acrylamide copolymer,a modified cross-linked polyvinyl alcohol, a neutralizedself-crosslinking polyacrylic acid, a crosslinked polyacrylate salt,carboxylated cellulose, and a neutralized crosslinked isobutylene-maleicanhydride copolymer.

Superabsorbent particles are available commercially, for example starchgraft polyacrylate hydrogel fines (IM 1000F) from Hoechst-Celanese ofPortsmouth, Va., or larger particles such as granules. Othersuperabsorbent particles are marketed under the trademarks SANWET(supplied by Sanyo Kasei Kogyo Kabushiki Kaisha), SUMIKA GEL (suppliedby Sumitomo Kagaku Kabushiki Kaisha and which is emulsion polymerizedand spherical as opposed to solution polymerized ground particles),FAVOR (supplied by Stockhausen of Greensboro, N.C.), and NORSOCRYL(supplied by Atochem). The superabsorbent particles come in a variety ofsizes and morphologies, for example IM 1000 and IM 1000F. The 1000F isfiner and will pass through a 200 mesh screen whereas IM 1000 hasparticles that will not pass through a 60 mesh screen. Another type ofsuperabsorbent particle is IM 5600 (agglomerated fines). Superabsorbentparticulate hydrophilic polymers are also described in detail in U.S.Pat. No. 4,102,340, which is incorporated herein by reference. Thatincorporated patent discloses hydrocolloid absorbent materials such ascross-linked polyacrylamides.

Other Particles

Many particles that form hydrogen bonds or coordinate covalent bonds aresuitable for use with the present invention. Some such particles arelisted in Table 1 with an indication of the function of the listedparticles.

                  TABLE I                                                         ______________________________________                                        Particulates for Binding                                                      Name            Function                                                      ______________________________________                                        Aluminum Trihydrate                                                                           Fire retardant, astringent                                    Acediasulfone   Antibacterial                                                 Agaricic acid   Antiperspirant                                                Alclometastone  Topical anti-inflammatory                                     Calcium alginate                                                                              Topical hemostatic                                            Amidomycin      Fungicide                                                     Antimony oxide  Fire retardant                                                Apigenin        Yellow dye, mordant                                           Arsenic disulfide                                                                             Red Pigment                                                   Aspirin         Anti-inflammatory; antipyretic                                Azanidazole     Antiprotozoal (Trichomonas)                                   Azelaic acid    Antiacne                                                      Baicalein       Astringent                                                    Bendazac        Anti-inflammatory                                             Benomyl         Fungicide; ascaricide                                         Benzestrol      Estrogen                                                      Benzylpenicillinic acid                                                                       Antibacterial                                                 Benzylsulfamide Antibacterial                                                 Bergaptene      Antipsoriatic                                                 Betasine        Iodine source                                                 Bezitramide     Narcotic analgesic                                            Bibrocathol     Topical antiseptic                                            Bietanautine    Antihistaminic                                                Bifenox         Herbicide                                                     Bifonazole      Antifungal                                                    Binapacryl      Fungicide, miticide                                           Bis(p-chlorophenoxy)                                                                          Miticide                                                      methane                                                                       Bismuth aluminate                                                                             Antacid                                                       Bismuth iodide oxide                                                                          Anti-infective                                                Bismuth phosphate                                                                             Antacid; protectant                                           Bismuth subcarbonate                                                                          Topical protectant                                            Bismuth subgallate                                                                            Astringent, antacid; protectant                               Bisphenol A     Fungicide                                                     Bitertanol      Agricultural fungicide                                        Bithionol       Topical anti-infective                                        Bromacil        Herbicide                                                     Bromadiolone    Rodenticide                                                   Bromcresol green                                                                              Indicator                                                     Bromcresol purple                                                                             Indicator                                                     Bromethalin     Rodenticide                                                   p-Bromoacetanilide                                                                            Analgesic; antipyretic                                        3-Bromo-d-camphor                                                                             Topical counterirritant                                       Bromophos       Insecticide                                                   Bromopropylate  Acaricide                                                     5-Bromosalicyl- antibacterial (tuberculostatic)                               hydroxamic acid                                                               5-Bromosalycilic acid                                                                         Analgesic                                                     acetate                                                                       Bromosaligenin  Anti-inflammatory                                             Bromthymol blue Indicator                                                     Broxyquinoline  Antiseptic; disinfectant                                      Bucetin         Analgesic                                                     Bumadizon       Analgesic; anti-inflammatory;                                                 antipyretic                                                   Bupirimate      Fungicide                                                     Busulfan        Carcinogen, insect sterilant,                                                 antineoplastic                                                Butamben        Topical anesthetic                                            Butrylin        Insecticide                                                   Butylated hydroxy-                                                                            Antioxidant (BHA)                                             anisole                                                                       Butyl paraben   Pharmaceutic aid; food                                                        preservative                                                  4-tert-Butylphenyl                                                                            Light absorber                                                salicylate                                                                    Cacotheline     Indicator                                                     Cactinomycin    Antineoplastic                                                Cadmium salycilate                                                                            Antiseptic                                                    Calamine        Skin protectant                                               Calcium carbonate                                                                             Antacid                                                       Calcium saccharate                                                                            Pharmaceutic aid                                              Calcium tartrate                                                                              Preservative; deodorant; antacid                              Cambendazole    Anthelminthic                                                 Candicidin      Topical antifungal                                            Candidin        Topical antifungal                                            Capsaicin       Topical analgesic                                             Captan          Fungicide; bacteriostat                                       Carbadox        Antimicrobial                                                 Carbamazepine   Anticonvulsant; analgesic                                     Carbarsone      Antiamebic                                                    Carbaryl        Contact insecticide                                           Carbazochrome   Antihemorrhagic                                               salycilate                                                                    Carbendazim     Fungicide                                                     Carbochloral    Hypnotic                                                      Carbophenothion Miticide; insecticide                                         Carboquone      Antineoplastic                                                Carisoprodol    Skeletal muscle relaxant                                      Carthamin       Dye                                                           Carvacrol       Disinfectant                                                  Cephalin        Local hemostatic                                              Chalcomycin     Antibiotic                                                    Chartreusin     Antibiotic                                                    Chitin          Vulnerary                                                     Chloramben      Herbicide                                                     Chloramphenacol Antimicrobial                                                 palmitate                                                                     Chloranil       Fungicide                                                     Chlorbetamide   Antiamebic                                                    Chlordimeform   Insecticide                                                   Chlorfenac      Herbicide                                                     Chlorfenethol   Acaricide                                                     Chlorhexidine   Topical antibacterial                                         Chloroazodin    Antibacterial; topical                                                        anesthetic                                                    Chlorophacinone Anticoagulant rodenticide                                     p-Chlorophenol  Antiseptic                                                    Chlorothricin   Antibiotic                                                    Chlorotrianisene                                                                              Estrogen                                                      Chloroxylenol   Antiseptic; germicide                                         Chlorphenesin   Topical antifungal                                            Chlorphenesin carbamate                                                                       Relaxant (skeletal muscle)                                    Chlorphenoxamide                                                                              Antiamebic                                                    Chlorpropamide  Antidiabetic                                                  Chlorpyrifos    Insecticide                                                   Chlorquinaldol  Topical antibacterial                                         Chlorsulfuron   Herbicide                                                     Chlorothion     Insecticide                                                   Chlozoxazone    Relaxant                                                      Cholesterol     Pharmaceutic aid                                              Chromic carbonate                                                                             Pigment                                                       Chromic hydroxide                                                                             Pigment                                                       Chromic oxide   Abrasive                                                      Chromic phosphate                                                                             Green pigment                                                 Chrysamminic acid                                                                             Explosive                                                     Chrysarobin     Antipsoriatic                                                 Cilastazol      Antithrombotic                                                Cinoxate        Sunscreen agent                                               ______________________________________                                    

Other suitable particles include proteins, vitamins, zeolites andsilica, which contain oxygen or nitrogen groups, or both. An example ofa suitable zeolite is Abscents odor absorber available from UOP ofTarrytown, N.Y. An example of a suitable antimicrobial particle ischlorhexidine(N,N"-Bis(4-chlorophenyl)-3,12-diimino-2,4,11,13-tetraazatetradecanediimidamide).The list in Table I is by no means exhaustive as it can be readilydetermined for each type of particle whether it is capable of forming ahydrogen bond or a coordinate covalent bond. Many of the particles arenon-absorbent, or not superabsorbent polymers.

The particles listed in Table 1 have chemical properties that make themsuitable for binding to fibers with the binders of the presentinvention. The listed particles are organic or inorganic compounds thathave little or no water solubility, yet have the capacity to hydrogenbond. Water solubility is preferably low, for example, less than 10 gdissolves completely in 300 ml of water at 25° C., more preferably lessthan about 1 g in 300 ml at 25° C. This low solubility allows theparticles to remain solid, and the hydrogen bonding capacity allows themto adhere to the fibers. Once bound, the particles substantially retaina discrete particulate form instead of dissolving or fusing. More of theparticles are discrete than fused once bound.

The amount of binder added to the fibers can vary widely, for examplefrom 0.05 to 80 percent of the total weight of the fibrous material,binders and particles. Antimicrobials such as chlorhexidine areeffective in very low amounts, such as 0.05 percent. Superabsorbentparticles are preferably added in an amount of 3-40 percent, especially15-25 percent by weight.

Polymeric Binder Characteristics

The particles may be bound to the fibers by a water soluble polymericbinder selected from a predetermined group of polymeric binders thateach have a hydrogen bonding functionality or coordinate covalent bondforming functionality on each repeating unit of the polymer. Inaccordance with the present invention, the predetermined groups ofpolymeric binders includes the set of binders consisting ofpolypropylene glycol (PPG); a PPG/PEG copolymer; polyacrylic acid; apoly(caprolactone) diol; a polyamide such as polyglycine or anotherpolypeptide; or a polyamine such as polyethyleneimine or polyvinylpyridine. As used herein, a polymer is a macromolecule formed bychemical union of 5 or more identical combining units (monomers). Apolyamine is a polymer that contains amine functional groups and apolyamide is a polymer that contains amide functional groups. Each ofthe binders has a hydrogen bonding or a coordinate covalent bondingfunctionality on each repeating unit (monomer) of the polymer. Thisrepeating functionality may be a hydroxyl, carboxylic acid, amide, etheror amine. These binders are capable of forming hydrogen bonds becausethey have a functional group that contains an oxygen or a nitrogen.

The polyglycol has repeating ether units with hydroxyl groups at theterminal ends of the molecule, and polyacrylic acid has a repeatingcarboxyl group in which a hydrogen is bound to an electronegativeoxygen, creating a dipole that leaves the hydrogen partially positivelycharged. The polyamide (such as a polypeptide) or polyamine has arepeating NR group in which a hydrogen may be bound to anelectronegative nitrogen that also leaves the hydrogen partiallypositively charged. The hydrogen in both cases can then interact with anoxygen or nitrogen on the particle or fiber to form a hydrogen bond thatadheres the binder to the particle and fiber. The electronegative oxygenor nitrogen of the binder can also form a hydrogen bond with hydrogenson the particle or fiber that have positive dipoles induced by oxygensor nitrogens to which the hydrogen is attached. The polyamide also has acarboxyl group with an electronegative oxygen that can interact withhydrogen atoms in the particles or fibers.

Thus, the polymeric binders enhance the hydrogen bonding (a) between thefibers and binder; and (b) in the case of particles with hydrogenbonding functionalities, between the binder and the particles.

Alternatively, the polymeric binder may form a coordinate covalent bondwith the particles and a hydrogen bond to the fibers. For example, theoxygen or nitrogen on the binder has an unbound pair of electrons thatcan be donated to an empty orbital in the particle to form a coordinatecovalent bond. For example, one free pair of electrons on the oxygen ornitrogen can be donated to the empty p orbital of a boron containingparticle to form a coordinate covalent bond that adheres the particle tothe binder. The fibers themselves contain functional groups that canform hydrogen bonds with the binder, and allow the binder to adhere tothe fiber. Cellulosic and synthetic fibers, for example, containhydroxyl, carboxyl, amide, ether and ester groups that will hydrogenbond with the hydroxyl, carboxylic acid, amide or amine groups of thebinder. Hence the polymeric binder will adhere the particle with acoordinate covalent bond and the fiber will adhere with a hydrogen bond.

In some preferred embodiments, the polymeric binder is bound to both thefibers and the particle by hydrogen bonds. A polypropylene glycolbinder, for example, can be used to bind polyacrylate hydrogel particlesto cellulosic fibers. The hydroxyl and ether groups on the glycol binderparticipate in hydrogen bonding interactions with the hydroxyl groups onthe cellulose fibers and the carboxyl groups on the polyacrylatehydrogel, as shown below: ##STR3## Hence the binder will adhere both theparticle and fiber with hydrogen bonds. The presence of a hydrogenbonding functionality on each repeating unit of the polymeric binder hasbeen found to increase the number of hydrogen bonding interactions perunit mass of polymer, which provides superior binding efficiency anddiminishes separation of particles from the fibers. The repeating etherfunctionality on the glycol binder provides this efficiency in theexample diagrammed above. A repeating carboxyl group is the repeatingfunctionality on polyacrylic acid, while repeating carbonyls and NRgroups (wherein R is either an H or alkyl, preferably lower alkyl i.e.less than five carbon atoms, with the alkyl normal or iso) of the amidelinkages are the repeating functionalities on polyamides such aspolypeptides. A repeating amine group is present on polyamines.

The polymeric organic binders of the present invention have been foundto increase in binding efficiency as the length of the polymerincreases, at least within the ranges of molecular weights that arereported in the examples below. This increase in binding efficiency isattributable to the increased number of hydrogen bonding or coordinatecovalent bonding groups on the polymer with increasing molecular length.Each of the polymeric binders has a hydrogen bonding or coordinatecovalent bonding functionality on each repeating unit of the polymer,hence longer polymers provide more hydrogen bonding groups or coordinatecovalent bonding groups that can participate in hydrogen bondinginteractions or coordinate covalent bonds.

Although the invention is not limited to polymeric binders of particularmolecular weights, polymeric binders having a molecular weight greaterthan 500 grams/mole are preferred because they provide attractivephysical properties, and the solid is less volatile and morethermoplastic as compared to small polymeric binders. Polymeric binderswith molecular weights greater than 4000 grams/mole are especiallypreferred, because they have minimal volatility and are less likely toevaporate from the fibers. In some particular embodiments, polymers withmolecular weights between 4000 and 8000 grams/mole have been used.Polymers with molecular weights above 8000 may be used, but exceedinglyhigh molecular weight polymers may decrease binding efficiency becauseof processing difficulties.

Certain polymeric binders have greater binding efficiency because theirrepeating functionality is a more efficient hydrogen bonding group. Ithas been found that repeating amide groups are more efficient thanrepeating carboxyl functionalities, which are more efficient thanrepeating hydroxyl functionalities, which in turn are more efficientthan amine or ether functionalities. Hence, polymeric binders may bepreferred that have repeating amine or ether functionalities, morepreferably repeating hydroxyl functionalities, and even more preferablyrepeating carbonyl or carboxyl functionalities, and most preferablyrepeating amide functionalities. Binding may occur at any pH, but issuitably performed at a neutral pH of 5-8, preferably 6-8, to diminishacid hydrolysis of the resulting fibrous product. Suitable binders maybe selected from the group consisting of polyethylene glycol;polyethylene glycol and polypropylene glycol, including copolymersthereof; polyethylene glycol, polypropylene glycol and polyacrylic acid;polyethylene glycol, polypropylene glycol, polyacrylic acid, and apolyamide; polyethylene glycol, polypropylene glycol, polyacrylic acid,a polyamide and a polyamine; polypropylene glycol alone; polypropyleneglycol and polyacrylic acid; polypropylene glycol alone; polypropyleneglycol, polyacrylic acid and a polyamide; and polypropylene glycol,polyacrylic acid, a polyamide and a polyamine; polyacrylic acid alone;polyacrylic acid and a polyamide; polyacrylic acid, a polyamide and apolyamine; a polyamide alone; a polyamide and a polyamine; or apolyamine alone. Poly(caprolactone) diol may optionally be a member ofany of these groups.

The group consisting of polyacrylic acid, polyamide and polyamine hasbeen found to have a especially good binding efficiency. Amongpolyamides, polypeptides are especially preferred.

Non-Polymeric Binder Characteristics

The particles may be bound to the fibers by a non-polymeric organicbinder selected from a predetermined group of binders that each have avolatility less than water. The vapor pressure of the binder may, forexample, be less than 10 mm Hg at 25° C., and more preferably less than1 mm Hg at 25° C. The non-polymeric binder has a functional group thatforms hydrogen bonds or coordinate covalent bonds with the particles. Inaccordance with the present invention, the predetermined group ofnon-polymeric binders may include a functionality such as an alcohol, acarboxylic acid, an aldehyde, an amino acid, an amide, or an amine,wherein each binder includes at least two such functionalities, and thetwo functionalities are the same or different. A requirement for thenon-polymeric binder is that it have a plurality of functional groupscapable of hydrogen bonding, or at least one group that can hydrogenbond and at least one group that can form coordinate covalent bonds. Asused herein, the term "non-polymeric" refers to a monomer, dimer,trimer, tetramer, and oligomers, although some particular non-polymericbinders are monomeric and dimeric, preferably monomeric.

Particularly preferred non-polymeric organic binders can form five orsix membered rings with a functional group on the particle surface. Anexample of such a binder is an amine or amino acid (for example, aprimary amine or an amino acid such as glycine) which forms six memberedrings by forming hydrogen bonds: ##STR4##

A six membered ring is also formed by the hydroxyl groups of carboxylicacids, alcohols, and amino acids. ##STR5##

A five membered ring can be formed by the binder and the functionalityon the surface of the particle, for example ##STR6## wherein theparticle is SAP and the binder is an alcohol, such as a polyol withhydroxyl groups on adjacent carbons, for example 2,3-butanediol.

Other alcohols that do not form a five membered ring can also be used,for example alcohols that do not have hydroxyl groups on adjacentcarbons. Examples of suitable alcohols include primary, secondary ortertiary alcohols.

Amino alcohol binders are alcohols that contain an amino group (--NR₂),and include binders such as ethanolamine (2-aminoethanol), diglycolamine(2-(2-aminoethoxy)ethanol)). Non-polymeric polycarboxylic acids containmore than one carboxylic acid functional group, and include such bindersas citric acid, propane tricarboxylic acid, maleic acid,butanetetracarboxylic acid, cyclopentanetetracarboxylic acid, benzenetetracarboxylic acid and tartaric acid. A polyol is an alcohol thatcontains a plurality of hydroxyl groups, and includes diols such as theglycols (dihydric alcohols) ethylene glycol, propylene glycol andtrimethylene glycol; triols such as glycerin (1,2,3-propanetriol); andpolyhydroxy or polycarboxylic acid compounds such as tartaric acid orascorbic acid (vitamin C): ##STR7## Hydroxy acid binders are acids thatcontain a hydroxyl group, and include hydroxyacetic acid (CH₂ OHCOOH)and lactic, tartaric, ascorbic, citric, and salicylic acid. Amino acidbinders include any amino acid, such as glycine, alanine, valine,serine, threonine, cysteine, glutamic acid, lysine, or β alanine.Non-polymeric polyamide binders are small molecules (for example,monomers or dimers) that have more than one amide group, such asoxamide, urea and biuret. Similarly, a non-polymeric polyamine binder isa non-polymeric molecule that has more than one amino group, such asethylene diamine, EDTA or the amino acids asparagine and glutamine.

Each of the non-polymeric binders disclosed above is capable of forminghydrogen bonds because it has a functional group that contains an oxygenor nitrogen, or has oxygen or nitrogen containing groups that include ahydrogen. The amino alcohol, amino acid, carboxylic acid, alcohol andhydroxy acid all have a hydroxyl group in which a hydrogen is bound toan electronegative oxygen, creating a dipole that leaves the hydrogenpartially positively charged. The amino alcohol, amino acid, amide andamine all have an NR group in which a hydrogen may be bound to anelectronegative nitrogen that also leaves the hydrogen partiallypositively charged. The partially positively charged hydrogen in bothcases then can interact with an oxygen or nitrogen on the particle orfiber that adheres the binder to the particle and fiber. Thepolycarboxylic acid, hydroxy acid, amino acid and amide also have acarboxyl group with an electronegative oxygen that can interact withhydrogen atoms in the particles and fibers. Similarly, electronegativeatoms (such as oxygen or nitrogen) on the fiber or particle can interactwith hydrogen atoms on the binder that have positive dipoles, andpartially positive hydrogen atoms on the fiber or particle can interactwith electronegative atoms on the binder.

Several hydrogen bonding interactions of two of the binders (glycine and1,3-propanediol) with cellulose are shown below: ##STR8##

The hydrogen bonding interactions are shown as dotted lines. One suchinteraction is shown between the nitrogen of glycine and a hydrogen ofan OH on cellulose. A hydrogen bond with glycine is also shown betweenan oxygen of the OH on glycine and the hydroxy hydrogen of an alcoholsidechain on cellulose. Hydrogen bonding interactions of the1,3-propanediol are shown in dotted lines between an oxygen on an OHgroup of the binder and a hydrogen of an OH group on the cellulosemolecule. Another hydrogen bond is also shown between a hydrogen on anOH group of the glycol binder and an oxygen in an alcohol sidechain ofthe cellulose.

Alternatively, the oxygen or nitrogen on the binder has an unbound pairof electrons that can be donated to an empty orbital in the particle toform a coordinate covalent bond. The free pair of electrons on theoxygen or nitrogen can be donated to the empty p, d or f orbital of aparticle (for example a boron containing particle) to form a coordinatecovalent bond that adheres the particle to the binder. The fibersthemselves do not normally contain functional groups that can formcoordinate covalent bonds with the binders, but hydrogen bondinginteractions allow the binder to adhere to the fiber. Cellulosic andsynthetic fibers, for example, contain hydroxyl, carboxyl and estergroups that will hydrogen bond with the hydroxyl, carboxylic acid, amideor amine groups of the binder. Non-cellulosic or non-synthetic fibersthat have these functionalities can also be used, for example silk,which has an amide linkage. Hence the binder will adhere the particlewith a coordinate covalent bond and the fiber with a hydrogen bond.

In some preferred embodiments, the binder is bound to both the fibersand the particle by hydrogen bonds. A polyol binder, for example, can beused to bind polyacrylate hydrogel particles to cellulosic fibers. Thehydroxyl groups on the polyol binder participate in hydrogen bondinginteractions with the hydroxyl groups on the cellulose fibers and thecarboxyl groups on the polyacrylate hydrogel. Hence the binder willadhere both the particle and fiber with hydrogen bonds. These hydrogenbonds provide excellent binding efficiency and diminish separation ofbound particles from the fibers.

A structural drawing is shown below in which citric acid, vitamin C andurea adhere polyacrylate particles to cellulose with hydrogen bonds.Some of the possible hydrogen bonding interactions are shown as dashedlines. ##STR9##

Particularly efficient hydrogen bonding binders include those withcarboxyl groups, such as ascorbic acid, or amide groups, such as urea.Hydroxyl groups are also very efficient binders. Amine and etherfunctionalities are less efficient binders.

Binders have functional groups that may be selected independently or incombination from the group consisting of a carboxylic acid, an alcohol,an amide and an amine, wherein the binder has at least two of thesefunctional groups, and each of the functional groups can be the same(for example, a polyol, polycarboxylic acid, glyoxal, and polyamine orpolyamide) or different (for example, an amino alcohol, hydroxyamide,carboxyamide, or amino acid). Functional groups may also be selectedindependently or in combination from the group consisting of acarboxylic acid alone; a carboxylic acid and an alcohol; a carboxylicacid, an alcohol and an amide; a carboxylic acid, an alcohol, an amideand an amine; an alcohol alone; an alcohol and an amide; an alcohol, anamide and an amine; an amide alone; an amide and an amine; and an aminealone. An aldehyde may optionally be a member of any of these groups.

Preferred functional groups for the non-polymeric binders may beselected independently or in combination from the group consisting of anamino alcohol, a polycarboxylic acid, a polyol, a hydroxy acid, an aminoacid, an amide, and a polyamine. Other preferred groups of bindersinclude an amino alcohol alone, an amino alcohol and a polycarboxylicacid, an amino alcohol, a polycarboxylic acid and a polyol; an aminoalcohol, a polycarboxylic acid, a polyol and a hydroxy acid; an aminoalcohol, a polycarboxylic acid, a polyol, a hydroxy acid and an aminoacid; an amino alcohol, a polycarboxylic acid, a polyol, a hydroxy acid,an amino acid and an amide; a polycarboxylic acid and a polyol; apolycarboxylic acid, a polyol and a hydroxy acid; a polycarboxylic acid,a polyol, a hydroxy acid, and an amino acid; a polycarboxylic acid, apolyol, a hydroxy acid, an amino acid and an amide; a polycarboxylicacid, a polyol, a hydroxy acid, an amino acid, an amide and a polyamine;a hydroxy acid and an amino acid; a hydroxy acid, amino acid and amide;a hydroxy acid, amino acid, amide and polyamine; an amino acid and anamide; an amino acid, amide and a polyamine; an amide and a polyamine;an amino alcohol alone, a polycarboxylic acid alone, a polyol alone, ahydroxy acid alone, an amino acid alone, an amide alone and a polyaminealone.

More specifically, the functionalities of the non-polymeric binder maybe selected from the group of glycerin (a polyol), ascorbic acid (apolycarboxylic acid and a hydroxy acid), glyoxal (a polyaldehyde), urea(a polyamide), glycine (an amino acid), pentaerythritol (a polyol), amonosaccharide, a disaccharide (a polyhydric alcohol), as well as citricacid, tartaric acid, dipropylene glycol, and urea derivatives such asDMDHEU. Subgroupings include glycerin; glycerin and ascorbic acid;glycerin, ascorbic acid and urea; glycerin, ascorbic acid, urea andglycine; glycerin, ascorbic acid, urea, glycine and pentaerythritol;glycine, ascorbic acid, urea, glycine, pentaerythritol and amonosaccharide; glycerin, ascorbic acid, urea, glycine, pentaerythritol,a monosaccharide and a disaccharide; ascorbic acid; ascorbic acid andurea; ascorbic acid, urea and glycine; ascorbic acid, urea, glycine andpentaerythritol; ascorbic acid, urea, glycine, pentaerythritol and amonosaccharide; ascorbic acid, urea, glycine, pentaerythritol,monosaccharide and a disaccharide; urea; urea and glycine; urea, glycineand pentaerythritol; urea, glycine, pentaerythritol and amonosaccharide; urea, glycine, pentaerythritol, a monosaccharide and adisaccharide; glycine; glycine and pentaerythritol; glycine,pentaerythritol and a monosaccharide; glycine, pentaerythritol, amonosaccharide and a disaccharide; pentaerythritol; pentaerythritol anda monosaccharide; pentaerythritol, a monosaccharide and a disaccharide;a monosaccharide; a monosaccharide and a disaccharide; and adisaccharide.

Process Advantages

The binders of the present invention also provide numerous processadvantages. Binding of particles to the fibers can occur, for example,without external application of heat. Hence particle binding may occurat ambient temperature if desired. The present invention is thereforedistinct from prior art crosslinking processes in which elevatedtemperatures are required to covalently crosslink cellulose groups toone another. Moreover, the binders of the present invention have theadvantage of being reactivatable by addition of a liquid solvent such aswater. Hence, a liquid binder (which would include a solution of a solidor liquid binder, or a binder that has a melting point below roomtemperature) can be applied to a cellulose mat in the absence of theparticles to be bound and the binder allowed to air dry, for exampleuntil it reaches an equilibrium moisture content with the moisture inthe ambient air. Alternatively, the binder can be applied as a solid,for example as particles or a powder. At a later stage of processing,water or another liquid is added to those portions of the mat whereparticulate binding is desired. The particles may then be added to themat and adhered to those portions of the mat that have been moistened.Alternatively, the particles may be added to the mat prior to activationof the binder.

The binders may be liquids at room temperature (such as glycerin), orliquid solutions of binders that are solids at room temperature (forexample, an aqueous solution of glycine), or liquid hot melts of solidbinders. Solid binders may be added to fibers in particulate form, forexample, by sprinkling binder particles on the fibers.

The binding reaction of the present invention can occur across a broadrange of pH without requiring a catalyst. A suitable pH range without acatalyst is 1-14, but preferred ranges are 5-8 or 6-8 because suchneutral pH ranges will produce fibrous products (such as celluloseproducts) that are less prone to damage by acid hydrolysis.

The moisture content of the fibers during the binding reaction is0.5-50%, suitably 5-40%, or preferably 5-20% water by weight of thefibers, binder and particle. A moisture content greater than 20%,preferably greater than 30%, or in the range 20-50%, or 30-50%, can beused even though such high moisture contents would interfere withintermediate anhydride formation and inhibit formation of covalent bondsin the production of high bulk fibers. Particles may be added to thefibers such that the particles are distributed throughout a fibrousproduct without being confined to a surface of the product. Theparticles can be distributed throughout the depth of a fiber productsuch as a mat or web.

The binder is suitably present in the treated product in an amount of atleast 3 percent and no more than 80 percent by weight of the fibers,particles, and binder ("percent by weight"). In especially preferredembodiments, the binder is present in an amount of 5-30 percent byweight. Below about 3 percent, an insufficient amount of binder ispresent to achieve adequate binding, while using excessive amounts ofbinder can introduce unnecessary expense into the binding process. Highpercentages of binder can also cause processing problems because thebinder material transfers to equipment surfaces.

Thermoplastic binders may also be used to help bind fibers to each otherand particles to fibers. The binder that has the hydrogen bonding orcoordinate covalent bonding functionalities may itself be thermoplastic.The polymeric binders of the present invention have the advantage ofbeing thermoplastic solids. Hence fibers treated in accordance with thepresent invention can be thermobonded by elevating the fiber temperatureabove the melting temperature of the binder to melt the thermoplasticbinder and thermoplastically bind the fibers to each other and thefibers to the particles. Alternatively, an auxiliary or second bindercan be applied to the fibers as a solid at room temperature, and thetemperature of the second binder elevated above its melting point tothermobond the fibers and particles. The auxiliary binder may be appliedto the fibers either before or after the primary binder is applied, butbefore thermobonding.

The binders of the present invention may be used with fibers that havesubstantial intrafiber covalent crosslinks (such as HBA available fromWeyerhaeuser) or fibers which are substantially free of infrafibercovalent crosslinking. Examples of individualized intrafiber crosslinkedfibers are seen in European Patent Applications 440 472 A1 and 427 317A2, which produce products that those publications describe as beingsubstantially free of interfiber bonds. Those fibers have beenindividualized and then cured in the presence of a crosslinking materialat an elevated temperature to produce high bulk fibers having intrafibercovalent crosslinks. The fibers of the present invention do not need tobe processed as in those European applications to eliminate interfiberbonds. Binders of the present invention can therefore be used withnatural fibers that have substantial interfiber bonding, which isdefined as fibers that have not been processed as in EuropeanApplications 440 472 A1 and 427 317 A2 to substantially eliminateinterfiber bonds. Cellulose fibers that have not been so processed aresubstantially free of intrafiber bonds.

Although intrafiber covalent crosslinking is not required for thepresent invention, such high bulk fibers can be used with the bindersdisclosed herein. Fibers that have high bulk from intrafiber covalentcrosslinks are prepared by individualizing the fibers and curing them atan elevated temperature (above 150° C.) in the presence of acrosslinking material such as citric acid. Initial application of thebinder on such high bulk fibers may occur after the curing step,particularly if the binder is capable of functioning as a crosslinkingmaterial. The binders disclosed herein that can also crosslink arepolyols, polycarboxylic acids, and polyamines (both polymeric andnonpolymeric binders that have more than one amine group); none of theother specifically disclosed binders are known to form covalent,intrafiber bonds. If crosslinking binders are present during curing, thebinder can be consumed during the curing step to form covalentlycrosslinked ester bonds. When this occurs, the binder is no longeravailable for hydrogen bonding, and particle binding to fibers isineffective.

The binder can be applied before curing, even if the binder is also acrosslinking material. If the binder is a crosslinking material, stepsare taken to inhibit formation of anhydride intermediates that arerequired for covalent bond formation. Anhydride formation can beinhibited, for example, by adding a sufficient amount of water to thefibers to inhibit anhydride formation without preventing all covalentcrosslinking from occurring. Inhibition of anhydride formation can occurwhen the fibers have 20% water by weight of the fiber and are cured at150° C. for 20 minutes. Hence, at least 20% water (preferably 30%) byweight of the fiber should be present, or 20-50% water. Higher amountsof water within this range are preferred when curing at temperatureshigher than 150° C., or for periods of time longer than 20 minutes.

The fibrous product of the present method (with or without intrafibercrosslinking) may further be densified by external application ofpressure. The densified product is compact, easily transported, and hassuperior absorbent properties as compared to nondensified products. Thepresent inventors have found that the binders of the present inventionproduce a product that can be easily densified. Easy densification isassociated with the hydrogen bonds and coordinate covalent bonds formedbetween the binder and the particles and fibers. The fibers areparticularly easily densified when at least 5% by weight of the fibers,particles, and binder, more preferably 10%, are particles adhered to thefibers by the hydrogen bonds and/or coordinate covalent bonds of thepresent invention.

In accordance with this invention, the binders may be applied to fibersbefore, subsequent, or simultaneously with addition of the particles.Simultaneous addition can be accomplished by two separate streams ofparticles and binder that are simultaneously directed at a fibroussubstrate, or alternatively merged immediately prior to impactingagainst the substrate.

Binding is performed under conditions that favor formation of hydrogenbonds or coordinate covalent bonds, and discourage formation of covalentbonds. Conditions that favor covalent bonds are those disclosed in U.S.Pat. No. 4,412,036 and U.S. Pat. No. 4,467,012 wherein particle andbinder would be laminated between tissue layers under high temperatureand pressure to form laminated adherent tissue layers. That patentteaches that minimal adhesion occurs at 200 pli (pounds per linear inch,as in a calendar press) if no external heat is supplied, but adhesionimproves as the reaction temperature increases. Improved adhesion of thetissue layers occurs because of enhanced covalent bonding as thetemperature increases.

Conditions that favor covalent bond formation are also shown in EuropeanPatent Applications 440 472 A1; 427 317 A2; 427 316 A2; and 429 112 A2.These European publications use polycarboxylic acid crosslinkers, andrequire elevated temperatures (for example above 145° C.) and acidicconditions (pH less than 7) to promote formation of intrafiber covalentester bonds and inhibit reversion of the ester bonds. The presentinvention, in contrast, can form hydrogen or coordinate covalent bondsbelow 145° C., below 100° C., and even at room temperature. The bindersof the present invention can also bind particles to fibers under neutralor alkaline conditions, i.e., at a pH above 7, but preferably at a pH of5-8 or 7-8.

The intrafiber covalent bond forming processes described in the aboveEuropean publications require formation of an anhydride that then reactswith a hydroxy group on cellulose to form a covalent ester bond. Thepresence of more than about 20% water by weight in the fibers interfereswith formation of the anhydride and inhibits covalent bond formation.Hence, in processes that use binders that are also crosslinkers(polycarboxylic acid, polyols and polyamines) as binders in the presentinvention, the fibers should contain at least 20% water by weight (morepreferably 30%) if the particles and binder are present in the fiberswhen curing occurs. The water inhibits covalent bond formation, andprevents all of the binder from being used to form covalent intrafibercrosslinks. Hence, some of the binder remains available to form thenon-covalent bonds with the particles and produce ease of densificationin fiber products made by the process of the present invention.

The present invention, in contrast, produces a product under conditionsthat favor formation of hydrogen or coordinate covalent bonds. Hence,the particles can be bound to the fibers in the absence of the externalapplication of heat or pressure. Particles may also be bound and theresulting fiber product densified, for example at less than 200 pli(about 8000 psi), or less than 100 pli (about 4000 psi), in the absenceof external application of heat to produce a product in which asubstantial portion of the particles are bound by non-covalent bonds(hydrogen or coordinate covalent bonds). A substantial portion ofparticles bound by non-covalent bonds means at least half of theparticles are bound by other than covalent bonds, for example byhydrogen or coordinate covalent bonds.

In yet other examples, particles may be bound in the absence of externalapplication of pressure, but at elevated temperatures.

In particularly preferred embodiments, the particles are substantiallyentirely bound to the fibers non-covalently.

Binding Examples for Polymeric Binders

Several examples are given below illustrating use of the polymericbinders of the present invention to attach superabsorbent particles tosouthern bleached kraft pulp.

EXAMPLE I

A 321 gram amount of NB-416 southern bleached kraft fluff obtained fromWeyerhaeuser Company may be air-entrained in a blender-like mixingdevice and 100 grams of poly(caprolactone) diol (average molecularweight 2000, supplied by Aldrich Chemical Company of Milwaukee, Wis.)dissolved in 100 ml of deionized water may be sprayed onto the fluff asa binder. Then 435 grams of starch graft polyacrylate hydrogel fines (IM1000F; supplied by Hoechst-Celanese of Portsmouth, Va.) may be added andmixed. The product may then be removed from the blender, and spread outin a fume hood to air dry overnight. The resulting product may then beairlaid on a small airlay line, from M & J Machines (of Horsens,Denmark) and thermobonded at 140° C. for one minute to produce a webcontaining 40% superabsorbent particles (SAP) attached to theindividualized fibers. This binder has a low melting point, henceraising the temperature to 140° C. melts the binder and allows it toflow over the fibers and particles to enhance hydrogen bondinginteractions and provide mechanical encapsulation that further binds thefibers and particles. This is an example of activating a solid binder byheating it, without liquid addition. A polypropylene glycol/polyethyleneglycol copolymer binder would also behave in this manner.

EXAMPLE II

A 321 gram amount of southern kraft fluff was air-entrained in ablender-like mixing device and 154 grams of a 65% solution ofpolyacrylic acid (average molecular weight=2,000; supplied by AldrichChemical Company of Milwaukee, Wis.) diluted with 100 ml of deionizedwater was sprayed onto the fluff. Then 435 grams of polyacrylatehydrogel (FAVOR 800 supplied by Stockhausen of Greensboro, N.C.) wasadded into the mixing device and mixed with the fluff and polyacrylicacid binder. The product was removed and spread out to dry and then fedto a hammermill with a three-eighths inch round hole screen and shuntedto a small airlay line to produce a web containing 40% SAP attached tothe individualized fibers.

EXAMPLE III

A 321 gram amount of southern bleached kraft fluff is air-entrained in ablender-like mixing device and 100 grams of polyglycine (molecularweight=5,000-15,000; supplied as a dry powder by Sigma Chemical Companyof St. Louis, Mo.) diluted with 100 ml of deionized water is sprayedonto the fluff. Then 435 grams of starch graft polyacrylate hydrogelfines (IM 1000F; supplied by Hoechst-Celanese of Portsmouth, Va.) isadded and mixed. The product is removed and spread out in a fume hood todry overnight. The resulting product is fed into a Fitz hammermill witha three-eighths inch round hole screen and shunted to a small M & Jairlay line to produce a web containing 40% SAP attached to the fibers.

EXAMPLE IV

A 321 gram amount of southern bleached kraft fluff is air-entrained in ablender-like mixing device and 200 grams of a 50% solution ofpolyethyleneimine (molecular weight=50,000-100,000; supplied by ICNBiomedicals, Inc. of Costa Mesa, Calif.), or polyvinyl pyridine issprayed on the fluff. Then 435 grams of starch graft polyacrylatehydrogel fines (IM 1000F; supplied by Hoechst-Celanese of Portsmouth,Va.) is added and mixed. The product is removed and spread out in a fumehood to dry overnight. The resulting product is fed into a Fitzhammermill with a three-eighths inch round hole screen and shunted to asmall M & J airlay line to produce a web containing 40% SAP attached tothe fibers.

The classes of polymeric binders that encompass those described inExamples I-IV are especially preferred over other multiple hydrogenbonding functionality polymers for a number of reasons. One importantreason is that their functionalities produce very strong, effectivehydrogen bonding. Other important reasons include their relative lack ofreactivity (as compared with polyaldehydes or polyisocyanates) and theirlow toxicity (again, as compared with polyaldehydes or polyisocyanates).

EXAMPLE V

As previously described, repetition of a hydrogen bonding group on eachrepeating unit of a polymer has been found to produce a binder thatprovides superior binding of particles to fibers, as compared topolymeric binders in which the hydrogen bonding functionality is notpresent on all the repeating units. This example shows the difference inbinding efficiency between a 20% carboxylated polymer and a 100%carboxylated polymer. A bound sample was prepared as in Example I usinga 20% carboxylated ethylene acrylic acid copolymer and a 100%carboxylated PAA. A sample of each was subjected to the same mechanicalagitation (to simulate machine processing required to make a web),screened through a descending series of sieves to remove unattached SAP,and subjected to an absorbent capacity test (less attached SAP wouldresult in a lower absorbent capacity). The result of the test wasmeasured by weighing the unabsorbed liquid (0.9% saline) from astandardized insult, hence a lower number indicates more liquid absorbedor higher absorbent capacity.

A sample of the 20% carboxylated polymer (15% of the total mix) gave abeaker test result of 19.5 grams. A similar sample of polypropyleneglycol would give a result of about 20.0 grams. However, the hydrogenbonding functionality of PPG is not as efficient as the carboxylfunctionality of PAA. A similar sample of polyacrylic acid (100%carboxyl functionality of PAA) gave a result of 11.3 grams. A comparisonof the 20% and 100% carboxylated polymers shows a substantial increasein SAP binding efficiency, as measured by an increase in absorbency ofthe product.

Non-Polymeric Binding Examples

Several examples are given below illustrating use of severalnon-polymeric organic binders of the present invention to attachsuperabsorbent particles to southern bleached kraft pulp.

EXAMPLE VI

A 3171 gram amount of southern bleached kraft fluff was air-entrained ina blender-like mixing device and 1000 grams of glycerin (96%, USP;supplied by Dow Chemical Co. of Midland, Mich.) diluted with 300 ml ofdeionized water was sprayed onto the fluff. Then 4348 grams of starchgraft polyacrylate hydrogel fines (IM 1000F; supplied byHoechst-Celanese of Portsmouth, Va.) was added to the mixing device andmixed with the fluff and binder. The material was then shunted into aflash tube dryer at 142° F., blown into a cyclone and fed into a Danwebairlay machine to form a web containing bound 40% IM 1000F that issubstantially immobile in the web because the particles are bound to thefibers instead of mechanically entrapped by the matrix.

EXAMPLE VII

A 900 gram amount of southern bleached kraft fluff pulp sheet wassprayed with a 50% solution of glycine (supplied as a dry powder byAldrich of Milwaukee, Wis.) so that the moisture content was 17-21% asthe sheet was fed into a Fitz hammermill fitted with a three eighthsinch hole screen. Starch graft polyacrylate hydrogel fines (IM 1000F;supplied by Hoechst-Celanese of Portsmouth, Va.) were simultaneouslyadded to the mill by a screw feed device, mixed with the fluff, shuntedto an M & J airlay forming machine and airlaid to form a web. The webthat resulted contained 20% SAP attached to the fibers substantiallyuniformly throughout the web without being confined to a surface of theweb.

EXAMPLE VIII

A 900 gram amount of southern bleached kraft fluff pulp sheet wassprayed with a 50% solution of pentaerythritol (supplied by Aldrich ofMilwaukee, Wis.) so that the moisture content was 17-21% as the sheetwas fed into a Fitz hammermill fitted with a three eighths inch holescreen. Starch graft polyacrylate hydrogel fines (IM 1000F; supplied byHoechst-Celanese of Portsmouth, Va.) were simultaneously added to themill by a screw feed device, mixed with the fluff, shunted to an M & Jairlay forming machine and airlaid to form a web. The web that resultedcontained 20% SAP attached to the fibers.

EXAMPLE IX

A 900 gram amount of southern bleached kraft fluff pulp sheet was fedinto a Fitz hammermill fitted with a three eighths inch hole screen. Thesheet was defiberized, shunted to an M & J airlay line, and airlaid intoa web. As the web emerged, target zones of the web were misted with a50% solution of lactose to raise the moisture content to 17-21%. Fivegram aliquots of starch graft polyacrylate hydrogel fines (IM 1000F;supplied by Hoechst-Celanese of Portsmouth, Va.) were subsequentlysifted onto the target zones. The web that resulted contained targetzones with 5 grams of SAP attached to the fibers of each target zone.Portions of the web that were not targeted for lactose application didnot adhere the particles well. This is an example of applying the binderto a target zone so that SAP primarily adheres to the target areas wherethe binder was applied. Target zone application of SAP can beadvantageous because it reduces the cost of the product to provide SAPonly in areas of a product where the SAP is needed, for example, thecrotch area of a diaper. Placement of SAP in the area where a liquidinsult is expected also decreases the necessity for wicking liquid to aSAP impregnated region. This is an advantage because the requirement forwicking can increase liquid leakage in an absorbent product such as adiaper.

EXAMPLE X

A 321 gram amount of southern bleached kraft fluff was air-entrained ina blender-like mixing device and 100 grams of glycerin (96%, USP;supplied by Dow of Midland, Mich.) diluted with 30 ml of deionized waterwas sprayed onto the fluff. 71 grams of Abscents (an odor absorbingzeolite supplied by UOP of Tarrytown, N.Y.) was then added and mixed inthe mixing device with the fibers and glycerin for 15 seconds until ahomogenous mixture was achieved. The material was then spread out in afume hood overnight to dry, airlaid into a web and tested forparticulate retention by an ash test. The pad so produced contained 7%particulate. The original addition amount should have produced 15%,hence 50% particle retention was observed. This compares favorably toparticulate retention with latex binders under similar conditions inwhich only about 3% of particles are retained.

Product Characteristics

The following examples illustrate how SAP retention, pad integrity,wettability, bulk and liquid retention are affected by the glycerinbinder of the present invention.

EXAMPLE XI

Superabsorbent particles were bound to cellulose fibers with a glycerinbinder, as described in Example I above. For purposes of comparison,superabsorbent particles were bound to a separate sample of cellulosefibers using a polyvinyl acetate (PVAc) binder that was about 3%carboxylated, that is only about 3% of the PVA monomers werecarboxylated. Binding was performed as in Example I, but PVAc wassubstituted for glycerin. A 100 gram sample of the glycerin and PVActreated fluff with attached SAP was fed into a fan that was connected bya hose to a small cyclone mounted on top of a material containment box.This was done in an effort to simulate forces of mechanical agitationthe fluff would encounter during the airlay process. After collection inthe material containment device, fiber with attached SAP was removed andweighed. A five gram sample of the fiber with attached SAP was thenplaced in a column of sieves with decreasing mesh sizes and subjected toa shaking and thumping action for ten minutes in order to furtherdislodge any poorly attached SAP. Unattached or poorly attached SAPsifted through screens having a range of 5-60 mesh, while the fiber withwell attached SAP remained on the 5 mesh screen.

A 2.00 gram sample of the fibers that remained near the top of the sievecolumn was then placed in a 75 ml sample of 0.9% saline for exactly oneminute. After that minute, the liquid that was not absorbed was pouredoff into a separate, tared beaker and weighed. The relative amounts ofliquid absorbed is indicative of the amounts of SAP bound to the fiber.Fiber retaining higher amounts of SAP tend to absorb more water and givea smaller amount of liquid not absorbed.

These results are shown in Table I:

                  TABLE I                                                         ______________________________________                                        Glycerin Binder                                                               Comparing SAP Retention with Glycerin and PVAc Binders                               Binder Beaker result                                                   ______________________________________                                               40-504 22.8 g                                                                 (PVAc)                                                                        3666H  22.0 g                                                                 (PVAc)                                                                        Glycerin                                                                              5.5 g                                                          ______________________________________                                    

Table I illustrates that the glycerin binder provides a product that hasan absorbency increase of 400% compared to the PVAc binder. Asubstantial portion of this improvement is believed to be due to betteradhesion between the fibers and SAP, such that the particles are notdislodged from the fibers.

EXAMPLE XII

Pad integrity was compared in fibrous products that used no binder and aglycerin binder at 7% and 11% by weight. Each of these binders was usedto bind SAP to fibers as in Example I, and properties of the pad weremeasured and are shown in Table II:

                  TABLE II                                                        ______________________________________                                        Tensile Results                                                               Sample    Basis Weight                                                                              Density    Tensile Index                                ______________________________________                                        Pad integrity (low density):                                                  NB-416    464 gsm     0.12 g/cc  0.257 Nm/g                                   (control)                                                                     NB-416/7% 437.6 gsm   0.126 g/cc 0.288 Nm/g                                   Glycerin                                                                      NB-416/11%                                                                              402.5 gsm   0.135 g/cc 0.538 Nm/g                                   Glycerin                                                                      Pad Integrity (high density):                                                 NB-416    482.1 gsm   0.218 g/cc 0.475 Nm/g                                   (control                                                                      NB-416/7% 460.7 gsm   0.219 g/cc 0.882 Nm/g                                   Glycerin                                                                      NB-416/11%                                                                              421.6 gsm   0.248 g/cc 1.536 Nm/g                                   Glycerin                                                                      ______________________________________                                    

The glycerin binder in this example produced a product that had a highertensile index than an untreated product. The increased tensile strengthwas especially enhanced in the densified product.

EXAMPLE XIII

The effect of binders on the wettability and bulk of fibers was testedusing the following fibers: NB-316 (a standard southern bleached kraftpulp with no binder); GNB 25% (a standard southern bleached kraft pulpwith 25% glycerin (entrained and sprayed); HBA (a high bulk intrafibercrosslinked fiber available from the Weyerhaeuser Company that containsintrafiber covalent crosslinks); and GHBA (HBA fibers treated with aglycerin binder) in amounts of 12.5% and 25% by weight. Results aregiven in Tables III and IV.

FAQ time was determined by airlaying a specific quantity (4.00 grams) ofthe fluff to be tested into a clear plastic tube that was fitted with ascreen at one end. The fluff and tube were then placed into a well inthe test device and a metal plunger was lowered onto the fluff and thepad's bulk calculated. Water then flowed from underneath the pad, passedthrough the screen and wicked up through the pad. Absorbency time wasmeasured from when the liquid makes contact with the bottom screen untilthe water completes an electrical circuit by contacting the foot of theplunger resting on top of the pad. Lower absorbency times indicatebetter absorbency. Since the absorption of the liquid by the pad wasaccompanied with some collapse of the pad's structure, the bulk of thewet pad was then recalculated. The amount of liquid absorbed was thenmeasured and a gram per gram capacity for the material was calculated.

Table III gives FAQ time as a measure of wettability. A lower FAQ timeindicates a product that is more absorbent and wicks faster. Table IVgives wet bulk of fibers and the adjusted bulk of the fibers. Theadjusted bulk is a calculated number obtained by dividing the bulk bythe actual percent of pulp in the sample.

                  TABLE III                                                       ______________________________________                                        Wettability                                                                   Fiber                FAQ time                                                 ______________________________________                                        NB-316                3.0 sec                                                 GNB 25%               3.2 sec                                                 HBA                  13.5 sec                                                 GHBA 12.5%            4.5 sec                                                 GHBA 25%              0.4 sec                                                 ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        Bulk                                                                          Fiber          Wet Bulk    Adjusted Bulk                                      ______________________________________                                        NB-316         12.7 cc/g   12.7 cc/g                                          GNB 25%        10.9 cc/g   14.5 cc/g                                          HBA            19.4 cc/g   19.4 cc/g                                          GHBA 12.5%     16.1 cc/g   18.4 cc/g                                          GHBA 25%       14.9 cc/g   19.9 cc/g                                          ______________________________________                                    

The low FAQ times (Table III) in the glycerin treated fibers (GNB, GHBA)show that wettability is as good as the untreated fiber (NB-316). TheGHBA 25% had significantly better wettability than untreated HBA. Bulkof glycerin treated fibers (Table IV) was not significantly decreased orchanged at all levels of glycerin binder on a fiber to fiber comparisonbasis.

EXAMPLE XIV

Liquid retention of bound fibers was determined and compared to fibersin which no binder was added. NB-316 is a pulp sheet available fromWeyerhaeuser Company in which no binder is used. HBA is described inExample X. HBA/Gly SAP was an HBA fiber that was bound with glycerin(12% binder, 48% fiber) and which contained 40% SAP particles.NB-316/Gly SAP is NB-316 fibers to which glycerin and SAP fibers wereadded.

The procedure for determining liquid retention was to weigh triplicatesmall portions (near 0.2 grams) of samples to the nearest 0.0001 gramand then heat seal the small portions inside an envelope of a heatsealable nonwoven tea bag. The samples were then immersed in an excessof 0.9% saline for thirty minutes, then drained by suspending them froma clip for fifteen minutes. The samples were weighed to determine theamount of liquid absorbed. The grams of liquid absorbed per gram ofsample was calculated and the samples were spun in a centrifuge for oneminute. The samples were then reweighed and a percent liquid retentionwas calculated.

Results are shown in the following Table V:

                  TABLE V                                                         ______________________________________                                        Liquid Retention (after centrifuge)                                           Fiber/Binder         % Retention                                              ______________________________________                                        NB-316/none          less than 1%                                             HBA/none             less than 1%                                             HBA/Gly SAP          23%                                                      NB-316/Gly SAP       31.5%                                                    ______________________________________                                    

The results in Table V illustrate that fibers that have SAP bound tothem retain liquid well, while fibers without SAP retain liquid poorly.The glycerin binders provided excellent adherence of SAP to the fibers.

EXAMPLE XV Auxiliary Binder

As previously described, an auxiliary binder can be used in addition tothe non-polymeric binders of the present invention. A 321 gram amount ofa southern bleached kraft fiber (NB-416, supplied by Weyerhaeuser) wasair entrained in a blenderlike mixing device and sprayed with 212.8grams of a polyvinylacetate latex (PN-3666H, supplied by H B Fuller ofMinneapolis, Minn.). While still mixing, 438 grams of a water swellablepolyacrylate hydrogel (Favorsab 800, supplied by Stockhausen ofGreensboro, N.C.) was added and the resulting mixture was then sprayedwith 100 grams of a 50% solution of glycerin (supplied by Dow ofMidland, Mich.). The blender was then stopped and the mixture wasvacuumed from the blender and placed in a fume hood to air dryovernight. The dried product was then airlaid into a 6" diameter pad ina laboratory padformer, pressed to a density of approximately 0.077g/cc, and thermobonded at 140° C. for thirty seconds. The resulting padshad 40% bound SAP and improved tensile strength as compared to untreatedfluff with SAP and as also compared to binder treated fluff with SAPwithout the auxiliary binder.

Tensile strength was highest with polyvinylacetate alone, followed by acombination of polyvinylacetate and glycerin, then glycerin alone.Lowest tensile strength was seen with no binder at all.

EXAMPLE XVI

Binders of the present invention may be used to bind particles to pulpfibers that contain synthetic thermobonding fibers. In this example,KittyHawk pulp (available from Weyerhaeuser Company) is a mixture ofNB316 southern bleached kraft and 22% polyethylene thermoplastic binderfibers. The KittyHawk pulp is used to produce a pulp web, with SAP boundto the fibers as described in Example III. The web with adhered SAP isthen passed through a thermobonder to soften the polyethylene fibers andfuse the fibers of the web to each other to increase web strength.

EXAMPLE XVII

Solid sample ¹³ C NMR spectra were obtained on cellulose fibers treatedwith ascorbic acid to bind SAP to the fibers. An NMR spectra was alsoobtained on L-ascorbic acid. In both cases, separate spectra wereacquired using recovery delays of 1 sec and 5 sec between acquisitions.

The peaks in the treated fiber spectrum were assigned readily to thecomponents: SAP polyacrylate carboxyl (185 ppm) and backbone (50-30 ppm)carbons; cellulose (106, 90, 84, 76, 73 and 66 ppm); and ascorbic acidring carbons C-1, C-2 and C-3 (175, 119 and 156/153 ppm, respectively);the other ascorbic acid carbons are in the cellulose region, two of thembeing resolved at 69 and 61 ppm. The ascorbic acid carbon chemicalshifts in this ternary mixture were essentially identical (±0.2 ppm) totheir values in pure ascorbic acid. This indicated that the ascorbicacid in the treated fibers had undergone no gross structural changes,such as total neutralization, oxidation or ring opening.

The signal-accumulation rates observed at the two different recoverydelay times showed that the proton spins in pure ascorbic acid relaxedafter excitation much more slowly than they did in the ternary mixture.As shown in the following table, slow relaxation yields higher signalstrength at the long recovery delay relative to the short one. The fastproton spin-lattice relaxation in the coated fibers indicated that theascorbic acid in this system is held more tightly in place (i.e., isless mobile) than in the bulk acid. The ascorbic acid is apparently heldtightly by one or both of the other two components, cellulose and SAP,and not by other ascorbic acid molecules.

If the bonding were purely ionic, involving ascorbate ion and an acrylicacid unit in the SAP, then the NMR of the treated fibers would show theascorbic acid in the salt form. NMR reference spectra were found of theacid and its salt in aqueous solution, and C-3 is seen to shiftdramatically on ionization of its OH group: 156 ppm in the acid to 176ppm in the salt. Thus, since the NMR spectrum of the ternary mixturecontains the peaks at around 156 ppm, the ascorbic acid in this systemis not ionized.

The infrared spectra, however, point to substantial disruption in thestructure of the ring OH groups, comparing pure ascorbic acid with thetreated fibers, with the ascorbic acid in the mixture resemblingascorbate salts in having some of the OH stretching bands missing.

Looking at acidities, ascorbic and polyacrylic acids have nearlyidentical pK_(a) values (4.2 vs 5, respectively). They are both typicalstrong organic acids with weak conjugate bases. Thus, there is nocompelling reason for one of these acids to be neutralized (ionized) bythe conjugate base of the other acid. Rather, there should be a strongtendency for an ascorbic acid and an acrylate ion to share a hydrogenion between them, resulting in a long hydrogen bond between partiallyionic ascorbic and acrylic acid units. This sharing of hydrogen ionswould certainly be reflected in the IR spectrum, yet satisfies the NMRdata by not invoking full ionization of ascorbic acid. The spectroscopicdata are fully consistent with a hydrogen bonding mechanism betweenascorbic acid and an acrylate unit in the superabsorber.

    ______________________________________                                        Acrylic Acid NMR Amplitude Ratios                                             at Different Recovery Delay Times.                                                         Signal Ratio, 5 sec/1 sec                                        Peak Freq., ppm                                                                              Treated Fibers                                                                            Pure Acid                                          ______________________________________                                        176            1.99        5.21                                               156            1.92        --                                                 153            1.80        5.35                                               119            2.10        4.26                                               ______________________________________                                    

EXAMPLE XVIII Fibers With Superabsorber and Ascorbic Acid InfraredAnalysis

Infrared transmission spectra of the untreated NB316 pulp, the treatedNB316 pulp, ascorbic acid, and the IM 100F superabsorber were prepared.Then, a subtraction spectrum representing the treated pulp minus theuntreated control was obtained.

Examination of that subtraction spectrum indicated several infraredbands that were obviously associated with the ascorbic acid. They wereevident at 1755, ˜1690 (shifted slightly from 1660-1670), 868, 821, and756 wavenumbers (cm⁻¹). However, several other bands that were prominentin the ascorbic acid spectrum were absent in that subtraction spectrum.They included the following: 3525, 3410, 3318, 1319, 1119, and 1026cm⁻¹.

The higher frequency bands (3300-3600 cm⁻¹) in ascorbic acid areindicative of bonded OH groups. The infrared bands at 1319, 1119, and1026 cm⁻¹ may also be associated with OH vibrations. Consequently, theIR suggested that the subtraction spectrum reflected primarily a loss ofthe OH groups that were attached directly to the ring. A likelypossibility is that the OH groups were replaced by sodium. The onlyother major band in the subtraction spectrum was located at 1589 cm⁻¹.This was probably due to the superabsorber C=O which had shifted to aslightly higher frequency (from 1562 cm ⁻¹).

EXAMPLE XIX Activation

The binders of the present invention have the advantage of beingactivatable by addition of liquid or by heating. "Activation" includesactivating a previously inactive binder (e.g., by adding liquid to asolid) or reactivating a previously active binder (e.g., by addingliquid to a dried liquid binder) on the fibers. Hence, a liquid bindercan be applied to cellulose fibers, loose or in another form, such as acellulose mat, in the absence of the particles to be bound. The binderis then dried or allowed to dry, for example until the binder and fiberreach an equilibrium moisture content with ambient air. Alternatively,the binder can be applied as a solid, for example, particles sprinkledonto a fiber mat. At a later stage of processing, a liquid such as wateris added to the fibers resulting in an activation of the binder. Theparticulates may then be added, and the binder secures the particulatesto the fibers. This subsequent processing of the fibers to attach theparticles can occur, for example, at a separate location from thelocation where the binder was applied to the fibers. Therefore,manufacturers of products can add particulates of interest (e.g.,superabsorbent particles or fibers; antimicrobial particles, etc.) atthe place of manufacture of the end products that incorporate thetreated fibers. Also, more than one type of particulate material may beadded, if desired.

It has also been found that some of the binders of the present inventioncan be reactivated by mechanical agitation. For example, glycerin bindermay be applied to fibrous cellulose. The glycerin binder may be allowedto dry overnight, and the fibers then mechanically agitated in thepresence of superabsorbent particles to reactivate the glycerin binderand bind the particles to the fibers. Mechanical agitation may takeplace, for example, in a defiberizer where a sheet or mat of glycerintreated cellulose fibers are defiberized while being intimately mixedwith SAP that is bound to the fibers by the mechanical agitation.

Binder Activation Examples

Binder reactivation in the present invention allows binder to be addedto fibers either before or after particles are added to the fibers. Thebinder is subsequently activated by addition of liquid, heat, oragitation, and particles are bound to the fibers. The particles may beadded to the fibers either before binder activation, after binderactivation, or simultaneous with activation. If SAP is to be added tocellulose fibers, for example, the binder may be applied to a pulp sheetwhich is subsequently fiberized. A liquid such as water may be added tothe pulp before or after fiberization, and SAP may be added before orafter water addition, or simultaneously with the water. If SAP is addedafter water addition, the SAP should be applied to the fibers prior tocomplete evaporation of the added water from the fibers.

Activation can be of all the fibers, or only portions of the fibers,such as target zones or portions of the mat where particulate binding isdesired. The particles may then be added to the mat and adhered to thetarget zones of the mat which have been activated. In some embodiments,the binder is applied as a solid and heated during a later processingstage to activate the binder by softening it such that it binds theparticles to the fibers. The particles may be added in a patterncorresponding to a desired distribution of particles in the fibrousmaterial. Most commonly, however, activation is accomplished bymoistening a targeted area of the product into which an inactive (dry ordried) binder has already been introduced.

In yet other embodiments, the binder is applied to the fibers and thenactivated by applying kinetic energy to the fibers. Neat polypropyleneglycol (MW 2000) binder, for example, may be sprayed on fibers andallowed to air dry. Desired particles are then added to the fibers asthe fibers are mechanically agitated in a blender or defiberizer tokinetically activate the binder and bind the particles to the fibers.For kinetic activation, the binder may be added as a liquid or a solidto the fibers. In the case of liquid addition, the liquid is allowed toair dry, and then reactivated by mechanically agitating the fibers andbinder. In the case of solid binder addition, the binder is applied as asolid, and then moistened (for example, to a total fiber moisturecontent of about 7%) and then mechanically agitated.

Activation of the binder may be performed prior to adding the particles,subsequent to adding the partices, or simultaneously with addition ofthe particles. Once the binder is activated, it adheres a substantialportion of the particles to the fibers, wherein "a substantial portion"refers to about half of the particles present in the fibers, at leastwhere the particles are not added in excess.

In embodiments in which the binder is applied to the fibers as a solid,the activating step can comprise applying a liquid to the fibers afterthe binder has been applied to the fibers, shortly before the binder isapplied to the fibers, or simultaneously with application of the binderto the fibers.

The activating step is preferably performed after the curing step iscomplete, if a curing step is to be performed.

The following example will illustrate several specific applications ofthe activation process, and are not intended to limit the invention tothe disclosed methods.

EXAMPLE XX

The method of Example I above could be modified such that the SAP is notadded until after the web is heated to 140° C. A solid polyethyleneglycol/polypropylene glycol copolymer could be substituted for thebinder of Example I, and it would melt well below 140° C., and in itsliquid form bind the SAP to the fibers. The SAP could be appliedrandomly across the heated product, or applied specifically to atargeted zone of the product where enhanced absorbency is specificallydesired.

EXAMPLE XXI

A southern kraft pulp sheet would be immersed or sprayed with 154 gramsof a 65% solution of polyacrylic acid diluted with 100 ml of deionizedwater. The sheet is then allowed to air dry overnight, heated in an ovenat 80° C. for thirty minutes and conditioned in a 50% relative humiditychamber overnight. The sheet is then misted with water to raise itsmoisture content to 17-20% as it is fed into a Fitz hammermill filledwith a three-eighths inch hole screen. Polyacrylate hydrogel particlesof FAVOR 800 supplied by Stockhausen would simultaneously be added tothe mill by a screw feed device, mixed with the fluff, shunted to an M &J airlay forming machine and airlaid to form a web containing bound SAPthroughout the web, i.e., without being confined to a surface of theweb. Mixing SAP throughout the fluff helps produce a product in whichSAP is homogeneously or randomly distributed, which diminishes problemsof gel blocking.

EXAMPLE XXII

900 grams of KittyHawk pulp sheet (from the Weyerhaeuser Co., containing22% synthetic fiber) is immersed in a 10% by weight solution ofpolyglycine for thirty minutes. The 5 inch wide sheet was then uncoiledon a lab bench to air dry overnight, heated in an oven at 80° C. forthirty minutes and conditioned in a 50% relative humidity chamberovernight. The sheet is fed into a Fitz hammermill fitted with athree-eighths inch hold screen, defiberized, shunted to an M & J airlayline, and airlaid into a web. As the web emerges, circular target zonesof the web are misted with water from a spray bottle to raise themoisture content to 17-21% in the target zone. Five gram aliquots ofstarch graft polyacrylate hydrogel fines (IM 1000F; supplied byHoechst-Celanese of Portsmouth, Va.) are subsequently sifted onto thetarget zones to yield a web with SAP bound in target zones. The SAP doesnot form a confluent layer, but is instead present in particulate formon and below the surface of the web.

EXAMPLE XXIII

A 900 gram amount of southern bleached kraft fluff pulp sheet is sprayedwith a 50% solution of polyglycine so that the moisture content is17-21% as the sheet is fed into a Fitz hammermill fitted with athree-eighths inch hole screen. Starch graft polyacrylate hydrogel fines(IM 10000F; supplied by Hoechst-Celanese of Portsmouth, Va.) aresimultaneously added to the mill by a screw feed device as the sheet isfed into the hammermill, mixed with the fluff, shunted to an M & Jairlay forming machine and airlaid to form a web. The fines areintimately mixed with the fluff in the fibermill and bound to the fibersby the polyglycine binder to produce a web with particles distributedthroughout its width, and not restricted to a superficial surface.

EXAMPLE XXIV

A 900 gram amount of a southern bleached kraft pulp sheet was immersedin a 2% by mass solution of ascorbic acid (supplied as a dry powder byAldrich Chemical Co. of Milwaukee, Wis.) for thirty minutes. The 5 inchwide sheet was then uncoiled on a lab bench to air dry overnight, heatedin an oven at 80° C. for thirty minutes and conditioned in a 50%relative humidity chamber overnight. The sheet was then gravimetricallydetermined to be about 7% by weight ascorbic acid. The sheet was mistedwith water to raise its moisture content to 17-20% as it was fed into aFitz hammermill fitted with a three eighths inch hole screen. Mistingwith water reactivated the binder prior to addition of superabsorbentparticles (SAP). Starch graft polyacrylate hydrogel fines (IM 1000Fsupplied by Hoechst-Celanese of Portsmouth, Va.) were added as SAP tothe hammermill by a screw feed device, mixed with the fluff, shunted toan M & J airlay forming machine (from Horsens, Denmark) and airlaid toform a web. The web that resulted contained 20% SAP attached to thefibers by the binder.

EXAMPLE XXV

A 900 gram amount of KittyHawk pulp sheet (from the Weyerhaeuser Co.,containing 22% synthetic fibers) was immersed in a 10% by weightsolution of urea (supplied by Aldrich of Milwaukee, Wis.) for thirtyminutes. The 5 inch wide sheet was then uncoiled on a lab bench to airdry overnight, heated in an oven at 80° C. for thirty minutes andconditioned in a 50% relative humidity chamber overnight. The sheet wasthen gravimetrically determined to be about 30% by weight urea. Thesheet was fed into a Fitz hammermill fitted with a three eighths inchhole screen, defiberized, shunted to an M & J airlay line, and airlaidinto a web. As the web emerged, the binder in the dried web wasreactivated by misting target zones of the web with deionized water in acircular pattern from a spray bottle to raise the moisture content ofthe web or the target zones to 17-21%. Five gram aliquots ofpolyacrylate hydrogel (FAVOR 800 supplied by Stockhausen of Greensboro,N.C.) were subsequently sifted onto each reactivated target zone. Theweb that resulted contained target zones with 5 grams of SAP attached tothe fibers in each target zone. Alternative spray patterns could beprovided by selecting spray heads or different control devices that mistdifferent patterns.

Thermoplastic Binders

An auxiliary binder may also be used to help bind fibers to each otherabove the melting point of the auxiliary binder. The auxiliary bindermay be a solid thermoplastic material that is applied to the fibers andsoftened by elevating the temperature during the binding step to abovethe softening temperature of the auxiliary binder. The auxiliary binderis thereby temporarily softened, rendered more fluid (which for purposesof convenience may be referred to as auxiliary binder melting) andsubsequently resolidified as the temperature cools, whichthermoplastically binds the fibers to each other, and the particles tothe fibers. The auxiliary binder may also contain a hydrogen bondingfunctionality that hydrogen bonds the particles to the fiber. Examplesof auxiliary binders that are thermoplastic and also contain hydrogenbonding groups include ethylene vinyl alcohol, polyvinyl acetate,acrylates, polycarbonates, polyesters and polyamides. Furtherinformation about the use of such auxiliary binders can be found in U.S.Pat. No. 5,057,166.

The auxiliary or second binder can be added to the fibers, either beforeor after a first binder, to help bind the fibers to each other andprovide additional binding between the fibers and particles. A suitablesecond binder would be a thermoplastic or thermosetting binder. In thecase of thermoplastic polymers, the polymers may be a material whichremains permanently thermoplastic. Alternatively, such polymers may be amaterial which is partially or fully crosslinkable, with or without anexternal catalyst, into a thermosetting type polymer. As a few specificexamples, suitable thermoplastic binders can be made of the followingmaterials

ethylene vinyl alcohol

polyvinyl acetate

acrylic

polyvinyl acetate acrylate

acrylates

polyvinyl dichloride

ethylene vinyl acetate

ethylene vinyl chloride

polyvinyl chloride

styrene

styrene acrylate

styrene/butadiene

styrene/acrylonitrile

butadiene/acrylonitrile

acrylonitrile/butadiene/styrene

ethylene acrylic acid

polyethylene

urethanes

polycarbonate

polyphenylene oxide

polypropylene

polyesters

polyimides

In addition, a few specific examples of thermoset binders include thosemade of the following materials:

epoxy

phenolic

bismaleimide

polyimide

melamine/formaldehyde

polyester

urethanes

urea

urea/formaldehyde

More than one of these materials may be used to treat the fibers. Forexample, a first coating or sheath of a thermoset material may be usedfollowed by a second coating of a thermoplastic material. Thesuperabsorbent particles or other particles are then typically adheredto the outer binder material. During subsequent use of the fibers tomake products, the thermoplastic material may be heated to its softeningor tack temperature without raising the thermoset material to its curingtemperature. The remaining thermoset material permits subsequent heatingof the fibers to cure the thermoset material during further processing.Alternatively, the thermoset material may be cured at the same time thethermoplastic material is heated by heating the fibers to the curingtemperature of the thermoset with the thermoplastic material also beingheated to its tack temperature.

Certain types of binders enhance the fire resistance of the treatedfibers, and thereby products made from these fibers. For example,polyvinyl chloride, polyvinyl dichloride, ethylene vinyl chloride andphenolic are fire retardant.

Surfactants may also be included in the liquid binder as desired. Othermaterials may also be mixed with the liquid binder to impart desiredcharacteristics to the treated fibers. For example, particulatematerial, such as pigments, may also be included in the binder forapplication to the fibers.

EXAMPLE XXVI

As previously described, an auxiliary binder can be used in addition tothe polymeric binders of the present invention. A 3210 gram amount ofsouthern bleached kraft binder (NB-416, supplied by WeyerhaeuserCompany) is air entrained in a blenderlike mixing device and sprayedwith 2128 grams of a polyvinyl acetate latex (PN-3666H, supplied by H.B. Fuller of Minneapolis, Minn.). While still mixing, 4073 grams of awater swellable polyacrylate hydrogel (IM 1000-60, supplied byHoechst-Celanese of Portsmouth, Va.) is added and the resulting mixtureis then sprayed with 1160 grams of a 50% solution of polypropyleneglycol (supplied by Union Carbide of Danbury, Conn.). The blender isthen stopped and the mixture was shunted into a flash tube dryer. Thedried product is then airlaid as a 16 inch wide web on a Danweb airlaymachine, pressed to a density of approximately 0.15 g/cc, andthermobonded at 140° C. for thirty seconds. The resulting web would have40% bound SAP and improved tensile strength (as compared to untreatedfluff with SAP).

Application of Binder

The binders of the present invention can be added to the fibers in anyconvenient manner. One such procedure is to spray the binder or binderson a web of the fibers that is conveyed past a sprayer on a conveyorbelt. Alternatively, loose fibers may be allowed to fall past a sprayer,or loose fibers may be moved on a conveyor belt past a sprayer. Theloose fibers may also be slurried with or immersed in binder. For solidbinders, blending of the fiber and binder may be accomplished or thebinder may simply be sprinkled onto or otherwise comingled with thefibers. The fibers may also be sprayed or immersed in the binder, orbinder particles may be applied thereto. These fibers can, while stillwet in the case of a liquid binder or following reactivation of a liquidor solid, be combined with the particles. The fibers can also be allowedto dry for later reactivation with a reactivation liquid and combinedwith the particles at that time. Particles may be added fromconventional volumetric feeders in a hammermill or from injectors on apaper making line.

One method for uniformly coating the fibers with a binder and adding theparticles is shown in U.S. Pat. No. 5,064,689, which is incorporatedherein by reference. However, the invention is not limited to anyspecific mechanism for combining the fiber, binder and particles.

Composite Absorbent Product

In accordance with the present invention, absorbent structures may bemade from the fibers, with the bound particulates, in accordance withthe present invention. These articles may be composite structures (e.g.,made of plural materials). For example, the articles may have a core ofplural types of fibers, or fiber layers, with or without coveringmaterials. These products are capable of absorbing significantquantities of water and other fluids, such as urine and body fluids.Such products include, but are not limited to, disposable diapers,sanitary napkins, incontinent pads, towels and the like.

As best shown in FIGS. 1 and 2, an absorbent towel 200 may have a core216 with a cover sheet 232 and a backing sheet 234. The core 216 may becomprised of fibers with the binders of the present invention andparticulate materials, such as superabsorbent particles secured to thefibers by the binder. The fibers that contain the binder may be blendedwith other fibers as well in the core. Cover sheet 232 is made of anysuitable material, including liquid permeable, nonwoven materials, whichwill readily permit the passage of liquid through the cover sheet to theabsorbent pad 216. The following list of liquid permeable materials isprovided by way of example only: nonwoven sheets of polypropylene,rayon, nylon fibers, polyester fibers, and blends thereof. Aspecifically preferred cover sheet material for wipes is a 70% rayon/30%polyester blend having a basis weight of 21.5 grams/m², available fromthe Scott Paper Company.

The backing sheet 234 may be, but is not necessarily, made of a liquidimpermeable material, including but not limited to, films ofpolyethylene, polypropylene and polyester and blends thereof along withnylon and polyvinyl chloride films. A specifically preferred backingsheet material is a polyethylene film from Dow Chemical Company.

FIGS. 1-2 illustrate an absorbent pad structure which may be formed fromfibers of the present invention, whether or not they are blended withother fibers. FIGS. 1 and 2 represent an absorbent pad having a heatembossed screen pattern. Pads having no pattern may also be used. A padhaving a cover sheet and a backing sheet may be formed, for example, byplacing a square fiber piece cut from the sheet onto a correspondingprecut backing sheet. A corresponding precut cover sheet is placed overthe top of the fiber on the backing sheet. This assembly may then beadhesively bonded.

With reference to FIGS. 3-6, absorbent structures in the form ofbandages or dressings are shown. In FIGS. 3 and 4, a bandage 410 forapplication to a wound to absorb blood and other bodily fluids is shown.An absorbent pad 216 (FIG. 4) is securely mounted to an exterior or padmounting surface 414 of a backing strip 416. Any suitable mounting orsecuring means may be used to affix pad 216 to the surface 414 of thestrip 416. However, it is preferable for surface 414 to be coated withan adhesive so that the pad 216 may be adhesively mounted in place. Anexemplary adhesive is ethylene vinyl acetate adhesive. It is alsodesirable for the overall surface 418 of backing strip 416 to be coatedwith a conventional adhesive. Surface 418 is the surface which isaffixed to the area of the skin surrounding the wound. Conventional"peel-back" tabs may be used to protect the adhesive coating and pad 216until the bandage is to be applied. This type of backing strip is wellknown in the art.

The backing strip 416 may be of any known flexible material suitable forapplication to the skin. It is preferable for the strip 416 to be of amaterial which is impermeable to the passage of liquid so that fluidfrom a wound is contained by the bandage. However, the strip 416 may beapertured or otherwise breathable to permit air to reach the wound topromote the healing process. A specific example of a suitable backingstrip 416 is a polyethylene film.

As in the other structures described, a variety of combinations ofantimicrobials and other particles may be used in such a bandage. Again,however, the particles are adhered securely in place when the particleshave a hydrogen bonding or a coordinate covalent bonding functionality,the fibers to which these particles are bound have a hydrogen bondingfunctionality, and wherein the binder is selected from the groupconsisting of a polypropylene glycol, a polypropyleneglycol/polyethylene glycol copolymer, polyacrylic acid, a polyamide, ora polyamine and the polymeric binder has a hydrogen bonding or acoordinate covalent bond forming functionality on each repeating unit ofthe binder. Two different particles, such as antimicrobials inparticulate form, may be adhered to the same fiber. In the alternative,each different type of antimicrobial particle or other particle may beadhered separately to different fibers. Also, blends of fibers may beincluded in absorbent structures such as pad 216. For example, theseblends may include fibers with adhered antimicrobial (one or moreantimicrobials) particles and adhered superabsorbent particles; fiberswith one or more antimicrobial particles without superabsorbentparticles blended with fibers having adhered superabsorbent particleswith or without antimicrobial particles; and combinations of such fiberswith untreated fibers and/or binder coated fibers without superabsorbentparticles or antimicrobial particles. In addition, other particles, suchas anticoagulants or hemostatics may be attached to the fibers.

The absorbent pad 216 of bandage 410 may also include a cover sheet 420.Cover sheet 420 is typically made of any suitable material which willreadily permit the passage of liquid through the cover sheet to theabsorbent pad 216, such as nonwoven fiber webs of fibers such as, forexample, rayon, nylon, polyester, propylene and blends thereof. Onespecifically preferred cover sheet material is a 70 percent rayon/30percent polyester blend having a basis weight of 18 g/m² from ScottPaper Company.

The dressing 216 shown in FIGS. 5 and 6 illustrates fibers 421 placedwithin an enclosure 422. Enclosure 422 has at least one liquid permeablesurface through which fluid or liquid may pass to be absorbed by thefibers. The enclosure containing the loose fibers may be secured to theskin using adhesive tape, for example. Again, the fibers 421 preferablyinclude antimicrobial particles attached to at least some of the fibers.

FIGS. 7 and 8 illustrate fibers of the present invention incorporatedinto a feminine hygiene appliance such as a feminine pad or tampon. Inthis case, the feminine pad 510 is illustrated as having a cover sheet512. The loose fibers having adhered antimicrobial particles, which mayalternatively be in the form of a pad, are included within the interiorof the feminine appliance as indicated at 216 in FIG. 2. The cover 512is preferably liquid permeable so that bodily fluids may reach theinterior of the pad for purposes of absorption. The cover 512 may bewrapped around the core 216 (as indicated by edges 514, 515). A backingsheet 516, preferably of a liquid impermeable material, may be adheredto the edges 514, 515 at the underside of the core. An adhesivecontaining strip, such as indicated at 520, which may have a peelable orremovable cover, may be mounted to the backing sheet 516 for use inadhering the pad, for example to a user's undergarment, during use.

FIGS. 9 and 10 illustrate a conventional disposable diaper 550 with acore 552 which is comprised of fibers of the present invention withadhered superabsorbent particulate materials. These particulatematerials may be confined to a target zone (for example, the frontportion of a diaper indicated at 556) or of a heavier concentration inthe target zone. This can be accomplished by airlaying fibers of thepresent invention in such a zone. Also, the core may be reactivated bymelting the binder or moistening the target zone with water. Thesuperabsorbent particles may be sprinkled on or otherwise applied tothis wetted zone. As the zone dries, the particles are adhered in place.

Densification

The products such as described above, as well as webs of the fibers ofthe present invention, can also be densified by external application ofpressure to the web. The web of Example II, for instance, could bedensified by passing it through a set of calendar rolls set at 60 and 90pli (pounds per linear inch, as in a calendar press) respectively toyield sheets with increased densities. Densification may alternativelybe provided by compaction rolls or presses. The present inventors havefound that densification is facilitated in the products treated with thepolymeric organic binders of the present invention. Products that aretreated with the binders of the present invention require less heat andpressure than untreated fibers to densify to a given density.Densification is preferably performed to produce a product that has adensity of about 0.1 to 0.7 g/cc, more preferably 0.1 to 0.3 g/cc.

An example of densification using some of the binders of the presentinvention is given below:

EXAMPLE XXVII

Any of the products of the present invention can be formed into 550gram/square meter sheets, six inches in diameter, in a laboratorypadformer. Those pads are then passed through a set of calendar rollsset at 60 and 90 pli, respectively to yield sheets with densities of 0.3and 0.5 g/cc.

EXAMPLE XXVIII

CCF pulp (Weyerhauser Company), containing 40% SAP by weight, was coatedwith 12.5% glycerin, air laid, and densified to 0.3 g/cc. Densificationto this product density required 60 psi for the glycerin coated fibers.In comparison, NB-316 fibers that were not coated with glycerin requireddensification at 200 psi to reach a 0.3 g/cc density. Absorbent capacitywas higher for the coated fibers. This comparison indicates that thefibers with glycerin bound SAP were more easily densified than thefibers that were not coated with the binder.

EXAMPLE XXIX

A 50 gram amount of polypropylene glycol is diluted with 50 gramsdeionized water. The resulting solution is sprayed on 321 grams of anintrafiber crosslinked cellulose fluff (HBA from Weyerhaeuser Company ofTacoma, Wash.) that was air entrained in a blender like mixing device.While the HBA fiber is still damp, 438 grams of IM 1000F (supplied byHoechst-Celanese, of Portsmouth, Va.) is added to the mixture. Theresultant mixture is then vacuumed from the blender and spread on acounter to dry overnight. Then 550 gram/square meter handsheets, sixinches in diameter, are made in a laboratory padformer. Those pads arethen pressed at 2000 and 3000 psi (or 60 and 90 pli in a calendar roll),respectively, to yield sheets with densities of 0.3 and 0.5 g/cc.Alternatively, pads of untreated HBA blended with 45% IM 1000F wouldrequire heating to 100° C. and pressures between 8,000 and 11,000 psi toproduce pads of similar densities.

Particulate Binding

FIG. 11 shows an isolated, enlarged cellulose fiber 600 with SAPparticles 602 bound to it by a binder of the present invention. Thisdrawing illustrates an example of the SAP retaining its discreteparticulate form following binding to the fibers. Some particle toparticle fusion may occur in accordance with this invention, butmaintenance of a discrete particulate form excludes formation of acompletely confluent film in which the particles lose their particulateidentity. Such a confluent film produces gel blocking that interfereswith efficient liquid absorption into the fibers.

The shown fiber 600 is elongated, and has an aspect ratio (ratio oflength to width) of about 10:1.

FIG. 12 shows the particles 602 distributed substantially uniformlythroughout the depth 604 of a pad 606. The particles are also shownadhering to all the surfaces of the pad. Particles may be distributed inany desired pattern throughout the pad in accordance with thisinvention, and need not necessarily adhere to all surfaces or bedistributed throughout the volume of the mat, or distributed uniformly.

Electron Photomicrographs

An electron photomicrograph of superabsorbent particles (SAP) bound tocellulose fibers with an ascorbic acid binder is shown in FIG. 13. TheSAP is at the left margin of the photograph, and is fused to the fiberwhich occupies the central portion of the photomicrograph. The particleis seen to be fused to the fiber, and the fiber has undergone some sheardamage that resulted in a fracture of the fiber. It is significant thatthe fiber has experienced shear damage while the particle has remainedfused to the fiber, because this indicates that the particle-fiber bondformed by the ascorbic acid is very strong and resilient, resistingmechanical disruption.

FIG. 14A-D shows several electron photomicrographs that illustrateindividual particles bound to fibers with a lactose binder. FIG. 14C,for example, shows that SAP retains its individual particulate form whenadhered to the fiber with a lactose binder. The particles do not form afused confluent mass without particulate identity.

Fiber Mixtures

The fibers of the present invention, such as fiber 600, can be mixedwith other types of fibers, such as that disclosed in U.S. Pat. No.5,057,166 which is incorporated herein by reference in its entirety. Thelatex coated fibers of that patent can be mixed with the fibers of thepresent invention to produce an absorbent product that hascharacteristics of both types of fibers.

Additional Binder Characteristics

U.S. Pat. No. 3,903,889 discloses a process for adhering absorbentparticles to pulp fibers using syrup, honey, and other polysaccharidessuch as dextrins. An essential requirement of these adhesive agents isthat they must possess the property of being permanently pliable, andnot rigidifying into a brittle film. The binders of the presentinvention, in contrast, are capable of functioning as a binder aftersolidifying into a rigid crystalline material. Even the binders of thepresent invention that do not rigidify into a solid (such as glycerin,low molecular weight PEG (below about 4000 g/mole) and PPG) are veryhygroscopic, and can be present on fibers having a total water contentof no more than 15%, or even 12%. This is in contrast to the adhesivessuch as honey and corn syrup disclosed in U.S. Pat. No. 3,903,889 thatare not hygroscopic. Polysaccharides (such as corn syrup, honey anddextrins) are excluded as binders from some embodiments of the inventionbecause they are a fertile substrate for microbial growth. Thepolysaccharide polymers are also excluded from nonpolymeric embodimentsof the binder. The non-polymeric saccharides, particularlymonosaccharide and disaccharide embodiments of the binder, lack the highviscosity of polysaccharides that gives them their tacky texture.

Having illustrated and described the principles of the invention in manypreferred embodiments, it should be apparent to those skilled in the artthat the invention can be modified in arrangement and detail withoutdeparting from such principles. We claim all modifications coming withinthe spirit and scope of the following claims.

We claim:
 1. A method of producing fibers with adhered particles,comprising:providing fibers that have hydrogen bonding functional sites;applying a binder to the fibers, the binder having a volatility lessthan water, the binder also having a functional group that is capable offorming a hydrogen bond with the fibers, and a functional group that iscapable of forming a hydrogen bond or a coordinate covalent bond withparticles that have a hydrogen bonding or a coordinate covalent bondingfunctionality; adding the particles to the fibers; and activating thebinder on the fibers from an inactive state, whereby a substantialportion of the particles are adhered in particulate form by the binderto the fibers by a hydrogen bond or coordinate covalent bond between theparticles and binder, and a hydrogen bond between the binder and fibers.2. The method according to claim 1 in which the binder is selected fromthe group consisting of (a) a polymeric binder with repeating units andwith each repeating unit having a functional group capable of forming ahydrogen bond or a coordinate covalent bond with the particles, or ahydrogen bond with the fibers; and (b) a non-polymeric organic binderwith a functional group capable of forming a hydrogen bond or acoordinate covalent bond with the particles, and a functional groupcapable of forming a hydrogen bond with the fibers.
 3. The methodaccording to claim 2 wherein the particles comprise superabsorbentparticles.
 4. The method according to claim 2 wherein the fiberscomprise wood pulp fibers.
 5. The method according to claim 2 whereinthe particles comprise superabsorbent particles and the fibers comprisewood pulp fibers.
 6. The method according to claim 2 wherein the binderis a polymeric binder selected from the group consisting of polyethyleneglycol, polypropylene glycol, poly(caprolactone) diol, polyacrylic acid,polyaldehydes, polyamides, polyamines, and copolymers thereof.
 7. Themethod according to claim 2 wherein the binder is water-soluble and isselected from the group consisting of polyacrylic acid, polyamides,polyamines, and copolymers thereof.
 8. The method according to claim 2wherein the binder is selected from the group consisting of a polyamideand a polyamine, and copolymers thereof.
 9. The method according toclaim 2 wherein the non-polymeric organic binder is water soluble andincludes a functionality selected from the group consisting of acarboxylic acid, an aldehyde, an alcohol, an amino acid, an amide, andan amine, and wherein there are at least two functionalities on themolecule selected from this group.
 10. The method according to claim 9wherein the binder includes a carboxylic acid functionality.
 11. Themethod according to claim 9 wherein the binder is selected from thegroup consisting of an amino acid, an amide, and an amine.
 12. Themethod according to claim 9 wherein the binder is selected from thegroup consisting of a polycarboxylic acid, a hydroxy acid, an amino acidand a carboxyamide.
 13. The method according to claim 9 wherein thebinder is an alcohol selected from the group consisting of a polyol andan amino alcohol.
 14. The method according to claim 9 wherein the binderincludes more than one amine functionality.
 15. The method according toclaim 9 wherein the binder is selected from the group consisting of ahydroxyamide and an organic binder that includes more than one amidefunctionality.
 16. The method according to claim 9 wherein the binder isselected from the group consisting of glycerin, ascorbic acid, urea,glycine, pentaerythritol, a monosaccharide, a disaccharide, citric acid,tartaric acid, dipropylene glycol, and urea derivatives.
 17. The methodaccording to claim 1 wherein the particles are selected from the groupconsisting of superabsorbent particles, zeolites, and antimicrobials.18. The method according to claim 1 in which the step of activating thebinder is performed prior to the step of adding the particles.
 19. Themethod according to claim 1 in which the step of activating the binderis performed subsequent to the step of adding the particles.
 20. Themethod according to claim 1 in which the step of activating the binderis performed simultaneously with the step of adding the particles. 21.The method according to claim 1 in which the step of activating thebinder comprises the step of adding an activating liquid to the fibersand binder to produce an activated binder, the activated binder adheringa substantial portion of the particles to the fibers.
 22. The methodaccording to claim 1 wherein the binder is a liquid and the activatingstep comprises activating the binder by mechanically agitating thefibers and binder, the binder then adhering a substantial portion of theparticles to the fibers.
 23. The method of claim 1 wherein the binder isapplied to the fibers as a solid, and the step of activating the bindercomprises applying a liquid to the fibers after applying the binder tothe fibers.
 24. The method of claim 1 wherein the step of activating thebinder comprises heating the fibers after applying the binder to thefibers.
 25. The method of claim 1 wherein the step of activating thebinder comprises applying kinetic energy to the fibers after applyingthe binder to the fibers.
 26. The method of claim 1 wherein the step ofapplying the binder comprises applying the binder to the fibers as aliquid binder and drying the binder liquid, and the step of activatingthe binder comprises reactivating the binder by applying a reactivationliquid to the fibers after the binder liquid has dried.
 27. The methodof claim 1 wherein the step of activating the binder comprisesactivating the binder in a pattern that corresponds to a desireddistribution of particles in the fibrous material.
 28. The method ofclaim 26 wherein the reactivation liquid is applied to the fibers in apattern that corresponds to a desired distribution of particles in thefibers.
 29. The method of claim 1 further comprising the step of curingthe fibers to form intrafiber covalent bonds, and the curing step isperformed before the activation step.
 30. The method of claim 1 whereinthe activating step comprises activating the binder and adhering theparticles without external application of heat.
 31. The method of claim9 wherein the binder is selected from the group consisting of glycerin,ascorbic acid, urea, glycine, pentaerythritol, citric acid, glyoxal,tartaric acid, dipropylene glycol, and urea derivatives.
 32. The methodof claim 31 wherein the binder is selected from the group consisting ofglycerin, ascorbic acid, urea, glycine, and pentaerythritol.
 33. Themethod of claim 9 wherein the binder is not a polysaccharide.
 34. Themethod of claim 9 wherein the binder is not a saccharide.
 35. The methodof claim 9 wherein the binder is not a tacky adhesive, and is capable offorming a rigid film upon air drying.
 36. The method of claim 9 whereinthe binder solidifies before the step of activating and is notpermanently pliable.
 37. A method of producing fibers with adheredparticles, comprising:providing cellulose fibers that have hydrogenbonding functional sites; applying a binder to the fibers, the binderhaving a volatility less than water, the binder also having a functionalgroup that is capable of forming a hydrogen bond with the fibers, and afunctional group that is capable of forming a hydrogen bond or acoordinate covalent bond with the particles, wherein the particles havea hydrogen bonding or a coordinate covalent bonding functionality, thebinder being selected from the group consisting of (a) a polymericbinder with repeating units and with each repeating unit having afunctional group capable of forming a hydrogen bond or a coordinatecovalent bond with the particles, or a hydrogen bond with the fibers;and (b) a non-polymeric organic binder with a functional group capableof forming a hydrogen bond or a coordinate covalent bond with theparticles, and a functional group capable of forming a hydrogen bondwith the fibers, the binder being present in an amount of at least 3% byweight of the fibers, particles and binder; adding the particles to thefibers in a sufficient amount that the binder when activated will bindthe particles as bound particles, such that the bound particles compriseat least 0.05% by weight of the fibers, particles and binder, whereinthe at least 0.05% bound particles are bound to the binder by hydrogenbonds or coordinate covalent bonds, and the binder to which the at least0.05% of particles are bound is in turn bound to the fibers by hydrogenbonds; and activating the binder by applying a sufficient amount ofliquid, heat or mechanical energy to bind the at least 0.05% ofparticles.
 38. The method of claim 37 wherein the step of activating thebinder comprises activating the binder under conditions that do notfavor formation of covalent bonds.
 39. The method of claim 38, whereinthe binder(a) is not a covalent crosslinker, or (b) the binder iscapable of covalently crosslinking cellulose in a crosslinking reactionupon heating the fibers at a temperature above about 150° C. in thepresence of the binder, thereby curing the fibers, and the curing isperformed under conditions that inhibit formation of covalent bonds andprevent the binder from being completely consumed in the crosslinkingreaction, such that a sufficient amount of binder remains fornoncovalently adhering the particles to the fibers after curing.
 40. Themethod of claim 39 wherein the binder is capable of covalentlycrosslinking the cellulose when the fibers are cured, and the activatingstep is performed subsequent to curing the fibers.